Semiconductor device and method for manufacturing semiconductor device

ABSTRACT

A semiconductor device with favorable electrical characteristics is provided. A semiconductor device with stable electrical characteristics is provided. 
     The semiconductor device includes a semiconductor layer, a first insulating layer, and a first conductive layer. The first insulating layer is provided over the semiconductor layer. The first conductive layer is provided over the first insulating layer. The semiconductor layer includes a first region that overlaps with the first conductive layer and the first insulating layer, a second region that does not overlap with the first conductive layer and overlaps with the first insulating layer, and a third region that overlaps with neither the first conductive layer nor the first insulating layer. The semiconductor layer contains a metal oxide. The second region and the third region contain a first element. The first element is one or more elements selected from boron, phosphorus, aluminum, and magnesium. The first element exists in a state of being bonded to oxygen.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C. §371 of International Application PCT/IB2019/051751, filed on Mar. 5,2019, which is incorporated by reference and claims the benefit offoreign priority applications filed in Japan on Mar. 16, 2018, asApplication No. 2018-048800 and Application No. 2018-048802.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice. One embodiment of the present invention relates to a displaydevice. One embodiment of the present invention relates to a method formanufacturing a semiconductor device or a display device.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention disclosed in this specification and the likecan include a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, an electronic device, alighting device, an input device, an input/output device, a drivingmethod thereof, and a manufacturing method thereof. A semiconductordevice refers to a device that can function by utilizing semiconductorcharacteristics in general.

BACKGROUND ART

As a semiconductor material applicable to a transistor, an oxidesemiconductor using a metal oxide has attracted attention. For example,Patent Document 1 discloses a semiconductor device that makesfield-effect mobility (simply referred to as mobility or μFE in somecases) to be increased by stacking a plurality of oxide semiconductorlayers, containing indium and gallium in an oxide semiconductor layerserving as a channel in the plurality of oxide semiconductor layers, andmaking the proportion of indium higher than the proportion of gallium.

A metal oxide that can be used for a semiconductor layer can bedeposited by a sputtering method or the like, and thus can be used for asemiconductor layer of a transistor included in a large display device.In addition, capital investment can be reduced because part ofproduction equipment for transistors using polycrystalline silicon oramorphous silicon can be retrofitted and utilized. Furthermore, atransistor using a metal oxide has high field-effect mobility comparedto the case of using amorphous silicon; therefore, a high-performancedisplay device provided with a driver circuit can be achieved.

In addition, Patent Document 2 discloses a thin film transistor in whichan oxide semiconductor film including a low-resistance region containingat least one kind in a group consisting of aluminum, boron, gallium,indium, titanium, silicon, germanium, tin, and lead as a dopant isapplied to a source region and a drain region.

REFERENCE Patent Documents

[Patent Document 1] Japanese Published Patent Application No. 2014-7399

[Patent Document 2] Japanese Published Patent Application No.2011-228622

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of one embodiment of the present invention is to provide asemiconductor device with favorable electrical characteristics. Anotherobject is to provide a semiconductor device with stable electricalcharacteristics. Another object is to provide a highly reliablesemiconductor device. Another object is to provide a highly reliabledisplay device.

Note that the description of these objects does not preclude theexistence of other objects. Note that one embodiment of the presentinvention does not need to achieve all the objects. Note that objectsother than these can be derived from the description of thespecification, the drawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention is a semiconductor deviceincluding a semiconductor layer, a first insulating layer, and a firstconductive layer. The first insulating layer is provided over thesemiconductor layer. The first conductive layer is provided over thefirst insulating layer. The semiconductor layer includes a first regionthat overlaps with the first conductive layer and the first insulatinglayer, a second region that does not overlap with the first conductivelayer and overlaps with the first insulating layer, and a third regionthat overlaps with neither the first conductive layer nor the firstinsulating layer. The semiconductor layer contains a metal oxide. Thesecond region and the third region contain a first element. The firstelement is one or more elements selected from boron, phosphorus,aluminum, and magnesium. The first element exists in a state of beingbonded to oxygen.

Another embodiment of the present invention is a semiconductor deviceincluding a semiconductor layer, a first insulating layer, a secondinsulating layer, and a first conductive layer. The first insulatinglayer is provided over the semiconductor layer. The first conductivelayer is provided over the first insulating layer. The second insulatinglayer is provided over the semiconductor layer, the first insulatinglayer, and the first conductive layer. The semiconductor layer includesa first region that overlaps with the first conductive layer and thefirst insulating layer, a second region that does not overlap with thefirst conductive layer and overlaps with the first insulating layer, anda third region that is in contact with the second insulating layer. Thesemiconductor layer contains a metal oxide. The second insulating layercontains more hydrogen than the first insulating layer. The secondregion and the third region contain a first element. The first elementis one or more elements selected from boron, phosphorus, aluminum, andmagnesium. The first element exists in a state of being bonded tooxygen.

One embodiment of the present invention is a semiconductor deviceincluding a semiconductor layer, a first insulating layer, a secondinsulating layer, and a first conductive layer. The first insulatinglayer is provided over the semiconductor layer. The first conductivelayer is provided over the first insulating layer. The second insulatinglayer is provided over the semiconductor layer, the first insulatinglayer, and the first conductive layer. The semiconductor layer includesa first region that overlaps with the first conductive layer and thefirst insulating layer, a second region that does not overlap with thefirst conductive layer and overlaps with the first insulating layer, anda third region that is in contact with the second insulating layer. Thesemiconductor layer contains a metal oxide. The second insulating layercontains one or more elements selected from aluminum, titanium,tantalum, tungsten, chromium, and ruthenium, and nitrogen. The secondregion and the third region contain a first element, and the firstelement is one or more elements selected from boron, phosphorus,aluminum, and magnesium. The first element exists in a state of beingbonded to oxygen.

In the above, the first insulating layer preferably includes a fourthregion that overlaps with the first conductive layer and the firstregion, and a fifth region that overlaps with the second region. In thatcase, the fifth region preferably contains the first element. In thatcase, the first element in the fifth region preferably exists in a stateof being bonded to oxygen.

In the above, the first insulating layer preferably contains an oxideand the second insulating layer preferably contains a nitride.

In the above, the first insulating layer preferably includes a portionprojected beyond a side surface of the first conductive layer, and anend portion of the first conductive layer is preferably located inwardfrom an end portion of the first insulating layer in a plan view.

In the above, the second insulating layer is preferably in contact witha top surface and a side surface of the first conductive layer, a topsurface and a side surface of the first insulating layer, and a topsurface and a side surface of the third region.

In the above, a second conductive layer and a third insulating layer arepreferably further included. In that case, it is preferable that thethird insulating layer be provided to cover the second conductive layer,the semiconductor layer be provided over the third insulating layer, andthe second conductive layer include a portion that overlaps with thesemiconductor layer, the first insulating layer, and the firstconductive layer with the third insulating layer therebetween.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including a first step in which a semiconductorlayer containing a metal oxide is formed, a second step in which a firstinsulating film containing an oxide and a first conductive film areformed over the semiconductor layer, a third step in which the firstconductive film and the first insulating film are etched to form a firstconductive layer and a first insulating layer including a portionprojected beyond a side surface of the first conductive layer and toform a portion of the semiconductor layer that is not covered with thefirst insulating layer, a fourth step in which a first element issupplied into the first insulating layer and the semiconductor layerusing the first conductive layer as a mask, and a fifth step in whichhydrogen is supplied to the portion of the semiconductor layer that isnot covered with the first insulating layer. The first element ispreferably boron, phosphorus, aluminum, or magnesium.

In the fourth step, the first element is preferably supplied by a plasmaion doping method or an ion implantation method.

In the fifth step, it is preferable that a second insulating layercontaining hydrogen be deposited in contact with the portion of thesemiconductor layer that is not covered with the first insulating layerby a plasma CVD method and then heat treatment be performed to supplyhydrogen to the semiconductor layer.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including a first step in which a semiconductorlayer containing a metal oxide is formed, a second step in which a firstinsulating film containing an oxide and a first conductive film areformed over the semiconductor layer, a third step in which the firstconductive film and the first insulating film are etched to form a firstconductive layer and a first insulating layer including a portionprojected beyond a side surface of the first conductive layer and toform a portion of the semiconductor layer that is not covered with thefirst insulating layer, a fourth step in which a first element issupplied into the first insulating layer and the semiconductor layerusing the first conductive layer as a mask, and a fifth step in which afirst layer is formed in contact with the portion of the semiconductorlayer that is not covered with the first insulating layer and then heattreatment is performed. The first element is boron, phosphorus,aluminum, or magnesium. The first layer preferably contains one or moreelements selected from aluminum, titanium, tantalum, tungsten, chromium,and ruthenium, and nitrogen.

In the fourth step, the first element is preferably supplied by a plasmaion doping method or an ion implantation method.

In the above, the heat treatment is preferably performed at atemperature higher than or equal to 200° C. and lower than or equal to450° C. in an atmosphere containing nitrogen.

Effect of the Invention

According to one embodiment of the present invention, it is possible toprovide a semiconductor device with favorable electricalcharacteristics. Alternatively, it is possible to provide asemiconductor device with stable electrical characteristics.Alternatively, it is possible to provide a highly reliable semiconductordevice. Alternatively, it is possible to provide a highly reliabledisplay device.

Note that the description of the effects does not preclude the existenceof other effects. Note that one embodiment of the present invention doesnot need to have all the effects. Note that effects other than these canbe derived from the description of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C A structure example of a semiconductor device.

FIGS. 2A to 2C A structure example of a semiconductor device.

FIGS. 3A to 3C Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 4A and 4B Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 5A and 5B Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 6A and 6B Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 7A and 7B Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 8A to 8C Diagrams illustrating a method for manufacturing asemiconductor device.

FIGS. 9A to 9C Top views of display devices.

FIG. 10 A cross-sectional view of a display device.

FIG. 11 A cross-sectional view of a display device.

FIG. 12 A cross-sectional view of a display device.

FIG. 13 A cross-sectional view of a display device.

FIGS. 14A to 14C A block diagram and circuit diagrams of a displaydevice.

FIGS. 15A to 15D Circuit diagrams and a timing chart of display devices.

FIGS. 16A and 16B A structure example of a display module.

FIGS. 17A and 17B A structure example of an electronic device.

FIGS. 18A to 18E Structure examples of electronic devices.

FIGS. 19A to 19G Structure examples of electronic devices.

FIGS. 20A to 20D Structure examples of electronic devices.

FIG. 21 Sheet resistance of metal oxide films.

FIG. 22 Sheet resistance of metal oxide films.

MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the drawings. Notethat the embodiments can be implemented with many different modes, andit will be readily understood by those skilled in the art that modes anddetails thereof can be changed in various ways without departing fromthe spirit and scope thereof. Therefore, the present invention shouldnot be construed as being limited to the description of embodimentsbelow.

Furthermore, in each drawing described in this specification, the size,the layer thickness, or the region of each component is exaggerated forclarity in some cases.

Furthermore, ordinal numbers such as “first,” “second,” and “third” usedin this specification are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In addition, in this specification, terms for describing arrangement,such as “over” and “under,” are used for convenience to describe thepositional relationship between components with reference to drawings.Furthermore, the positional relationship between components is changedas appropriate in accordance with a direction in which the componentsare described. Thus, terms for the description are not limited to termsused in the specification, and description can be made appropriatelydepending on the situation.

Furthermore, in this specification and the like, functions of a sourceand a drain of a transistor are sometimes interchanged with each otherwhen a transistor of opposite polarity is employed or the direction ofcurrent is changed in circuit operation, for example. Therefore, theterms “source” and “drain” can be used interchangeably.

Note that in this specification and the like, a channel length directionof a transistor refers to one of the directions parallel to a straightline that connects a source region and a drain region in the shortestdistance. In other words, a channel length direction corresponds to oneof the directions of current flowing through a semiconductor layer whena transistor is in an on state. In addition, a channel width directionrefers to a direction orthogonal to the channel length direction. Notethat each of the channel length direction and the channel widthdirection is not fixed to one direction in some cases depending on thestructure and the shape of a transistor.

Furthermore, in this specification and the like, “electricallyconnected” includes the case where connection is made through an “objecthaving any electric action.” Here, there is no particular limitation onthe “object having any electric action” as long as electric signals canbe transmitted and received between the connected components. Examplesof the “object having any electric action” include a switching elementsuch as a transistor, a resistor, an inductor, a capacitor, and otherelements with a variety of functions as well as an electrode and awiring.

Furthermore, in this specification and the like, the term “film” and theterm “layer” can be interchanged with each other. For example, in somecases, the term “conductive layer” and the term “insulating layer” canbe interchanged with the term “conductive film” and the term “insulatingfilm,” respectively.

Furthermore, unless otherwise specified, off-state current in thisspecification and the like refers to drain current of a transistor in anoff state (also referred to as a non-conducting state or a cutoffstate). Unless otherwise specified, an off state refers to, in ann-channel transistor, a state where voltage V_(gs) between its gate andsource is lower than the threshold voltage V_(th) (in a p-channeltransistor, higher than V_(th)).

In this specification and the like, a display panel that is oneembodiment of a display device has a function of displaying (outputting)an image or the like on (to) a display surface. Thus, the display panelis one embodiment of an output device.

Furthermore, in this specification and the like, a substrate of adisplay panel to which a connector such as an FPC (Flexible PrintedCircuit) or a TCP (Tape Carrier Package) is attached, or a substrate onwhich an IC is mounted by a COG (Chip On Glass) method or the like isreferred to as a display panel module, a display module, or simply adisplay panel or the like in some cases.

Note that in this specification and the like, a touch panel that is oneembodiment of a display device has a function of displaying an image orthe like on a display surface and a function of a touch sensor capableof sensing the contact, press, approach, or the like of a sensing targetsuch as a finger or a stylus with or to the display surface. Therefore,the touch panel is one embodiment of an input/output device.

A touch panel can also be referred to as, for example, a display panel(or a display device) with a touch sensor or a display panel (or adisplay device) having a touch sensor function. A touch panel caninclude a display panel and a touch sensor panel. Alternatively, a touchpanel can have a function of a touch sensor inside a display panel or ona surface thereof.

Furthermore, in this specification and the like, a substrate of a touchpanel to which a connector or an IC is attached is referred to as atouch panel module, a display module, or simply a touch panel or thelike in some cases.

Embodiment 1

In this embodiment, a semiconductor device and a display device of oneembodiment of the present invention and manufacturing methods thereofwill be described.

One embodiment of the present invention is a transistor including, overa formation surface, a semiconductor layer in which a channel is formed,a gate insulating layer (also referred to as a first insulating layer)over the semiconductor layer, and a conductive layer (also referred toas a first conductive layer) functioning as a gate electrode over thegate insulating layer. The semiconductor layer preferably contains ametal oxide exhibiting semiconductor characteristics (hereinafter alsoreferred to as an oxide semiconductor).

The gate insulating layer is preferably provided to cover part of thetop surface of the semiconductor layer. The gate electrode is preferablyprovided such that an end portion thereof is located inward from an endportion of the gate insulating layer. In other words, the gateinsulating layer preferably includes a portion projected beyond the endportion of the gate electrode, at least over the semiconductor layer.

The semiconductor layer includes a first region that overlaps with thegate electrode and the gate insulating layer, a second region thatoverlaps with the gate insulating layer and does not overlap with thegate electrode, and a third region that overlaps with neither the gateelectrode nor the gate insulating layer. The first region is a regionfunctioning as a channel formation region. The third region is a regionhaving lower resistance than the first region and a region functioningas a source region or a drain region. The second region is a regionhaving lower resistance than the first region, and preferably havinghigher resistance than the third region.

The second region can function as a buffer region for preventing carriersupply sources contained in the third region from diffusing into thefirst region, which is the channel formation region. When the secondregion is provided, the carrier concentration of the first region, whichis the channel formation region, can be made extremely low. The secondregion may function as an LDD (Lightly Doped Drain) region.

Here, the second region and the third region preferably contain anelement (also referred to as a first element) that may be bonded tooxygen in the metal oxide to cause an oxygen vacancy in the metal oxide.Boron, phosphorus, aluminum, magnesium, silicon, or the like can besuitably used as such an element, for example. It is further preferablethat the element exist in a state of being bonded to oxygen in thesemiconductor layer.

The elements in the second region and the third region are bonded tooxygen in the metal oxide, whereby oxygen vacancies are generated in themetal oxide. When the oxygen vacancies are bonded to hydrogen containedin the film, carrier supply sources are formed; thus, the second regionand the third region are each in a state of having lower resistance thanthe first region. Furthermore, the third region preferably has higherconcentration of the element than the second region. Accordingly, thethird region can be in a state of having lower resistance than thesecond region.

In addition, it is preferable that the first element be contained alsoin the gate insulating layer in a portion in contact with the secondregion, i.e., a portion that does not overlap with the gate electrode.Furthermore, it is preferable that the first element not be added to thegate insulating layer in a portion in contact with the channel formationregion, i.e., a portion that overlaps with the gate electrode.

When heat treatment is performed in a state where the gate insulatinglayer containing an oxide is provided in contact with the top surface ofthe semiconductor layer, oxygen released from the gate insulating layercan be supplied to the semiconductor layer. Thus, oxygen vacancies inthe semiconductor layer can be filled, and a highly reliable transistorcan be obtained.

Meanwhile, when oxygen is supplied to the second region, carrier densitydecreases and electrical resistance increases in some cases. However, inone embodiment of the present invention, the first element is added tothe portion of the gate insulating layer that is in contact with thesecond region. When the first element is added to an oxide film fromwhich oxygen can be released by heating, the amount of released oxygencan be reduced. As a result, supply of oxygen from the gate insulatinglayer to the second region can be inhibited, and the second region canmaintain the low-electrical-resistance state.

Employing such a structure makes it possible to achieve a semiconductordevice with excellent electrical characteristics and high reliability,which includes a channel formation region with sufficiently reducedoxygen vacancies and extremely low carrier density, a source region anda drain region that have extremely low electrical resistance, and an LDDregion therebetween.

Such a transistor can be manufactured by, for example, heat treatmentafter treatment for supplying the first element to the gate insulatinglayer and the semiconductor layer using the gate electrode as a mask.

At this time, the first element is further preferably supplied by aplasma ion doping method or an ion implantation method. These methodscan easily adjust the depth at which ions are added and thus make iteasy to add ions aiming at a region including the gate insulating layerand the semiconductor layer.

When the first element is added, ion supply conditions are preferablyset such that the concentration of the first element becomes the highestin a region of the semiconductor layer on the gate insulating layer sideor in the vicinity of an interface between the semiconductor layer andthe gate insulating layer. In that case, the first element atappropriate concentration can be added to both the semiconductor layerand the gate insulating layer by one step. Moreover, by formation of aregion having high concentration of the first element in a portion ofthe gate insulating layer near the second region, diffusibility ofoxygen in this portion is effectively reduced, and oxygen in the gateinsulating layer can be further effectively inhibited from diffusinginto the second region side. Furthermore, since the gate insulatinglayer is not provided over the third region, the first element is addedto the third region at high concentration to further reduce resistance;thus, contact resistance between the low-resistance region and a sourceelectrode or a drain electrode can be further reduced.

In addition, in the case where an element that is likely to be bonded tooxygen is used as the first element as described above, the firstelement exists in a state of being bonded to oxygen in the semiconductorlayer. In other words, the first element takes oxygen in thesemiconductor layer away to cause an oxygen vacancy in the semiconductorlayer, the oxygen vacancy is bonded to hydrogen in the film, and thus, acarrier is generated. Furthermore, the first element in thesemiconductor layer exists stably in an oxidized state; thus, the firstelement is not desorbed by heat or the like applied during themanufacturing process, which makes it possible to achieve alow-resistance region stably. For example, even when a temperaturehigher than or equal to 400° C., higher than or equal to 600° C., orhigher than or equal to 800° C. is applied during the manufacturingprocess, a stable low-resistance region can be maintained.

In addition, an insulating film containing an oxide is preferably usedas the gate insulating layer. Furthermore, oxygen that is released byheating (also referred to as excess oxygen) is preferably contained inthe gate insulating layer. In that case, the first element in the gateinsulating layer exists in a state of being bonded to excess oxygen inthe gate insulating layer. When the first element is bonded to excessoxygen to be stabilized, oxygen is hardly released from a region towhich the first element is added even when heating is performed, oroxygen does not easily diffuse. Accordingly, oxygen is supplied to thechannel formation region (first region) while preventing an increase inresistance due to supply of oxygen from the gate insulating layer to thesecond region, so that oxygen vacancies can be reduced. As a result, atransistor that has favorable electrical characteristics and highreliability can be obtained.

As the first element, an element that is stabilized by being bonded tooxygen in the semiconductor layer and in the gate insulating layer ispreferably used. For example, an element an oxide of which can exist asa solid in a normal state is preferably used. A particularly preferableelement can be selected from a typical non-metal element other than arare gas and hydrogen, a typical metal element, and a transition metalelement. It is particularly preferable to use boron, phosphorus,aluminum, magnesium, silicon, or the like.

By the way, a technique in which a dopant is supplied to a silicon thinfilm or bulk to make the silicon n-type or p-type is known. Thistechnique is a method for adjusting carrier density by substitution of adopant serving as a donor or an acceptor for a site of a silicon atom.For example, phosphorus or arsenic in silicon functions as a donor andthus can impart n-type conductivity, and boron or aluminum in siliconfunctions as an acceptor and thus can impart p-type conductivity. Inthis manner, the polarity of conductivity of silicon can be controlleddepending on a dopant type.

Meanwhile, in one embodiment of the present invention, the first elementextracts oxygen in the metal oxide so that an oxygen vacancy isgenerated in the film; when the oxygen vacancy and hydrogen in the filmare bonded to each other, a carrier is generated. Thus, the firstelement itself is not required to behave as a donor or an acceptor inthe metal oxide. Even an element that functions as an acceptor insilicon, such as boron or aluminum, for example, can impart n-typeconductivity to the metal oxide like phosphorus or the like. Hence, afunction required for the first element is completely different from afunction required for a dopant in silicon.

The third region preferably contains much hydrogen than the first regionand the second region. In that case, the third region can be in a stateof having further lower resistance than the second region. Even when thethird region contains much hydrogen, the second region can effectivelyinhibit diffusion of hydrogen into the first region. Accordingly, thechannel formation region has extremely low carrier density and thesource region and the drain region are brought into an extremelylow-resistance state; thus, a transistor with excellent electricalcharacteristics can be obtained.

As a method for supplying hydrogen into the third region, for example,heat treatment which is performed in a state of providing a filmcontaining hydrogen (also referred to as a second insulating layer) overand in contact with the third region is preferable. In that case, astructure is formed in which the first insulating layer is provided incontact with the top surface of the second region, and the second regionand the second insulating layer are not in contact with each other;thus, the hydrogen concentration of the second region can be lower thanthat of the third region.

Alternatively, hydrogen may be supplied to the semiconductor layer by amethod such as an ion doping method, an ion implantation method, or heattreatment in an atmosphere containing hydrogen, using the gate electrodeas a mask. Even in that case, the concentration of hydrogen supplied tothe second region can be lower than that of the third region because thefirst insulating layer is provided over the second region.

That is, the second region is a region having lower concentration of thefirst element than the third region, a region containing a smalleramount of oxygen vacancies than the third region, and a region havinglower hydrogen concentration than the third region; accordingly, thesecond region can be regarded as a high-resistance region having lowercarrier concentration than the third region.

Alternatively, the third region is preferably a region whose resistanceis reduced by performing heat treatment in a state of forming a firstlayer that covers the third region.

For the first layer, a film containing at least one of metal elementssuch as aluminum, titanium, tantalum, tungsten, chromium, and rutheniumcan be used. It is particularly preferable to contain at least one ofaluminum, titanium, tantalum, and tungsten. Alternatively, a nitridecontaining at least one of these metal elements or an oxide containingat least one of these metal elements can be suitably used.

For example, a nitride film such as an aluminum nitride film, analuminum titanium nitride film, or a titanium nitride film, or an oxidefilm such as an aluminum titanium oxide film can be suitably used.Alternatively, a metal film such as a tungsten film or a titanium filmmay be used.

In the case of using an aluminum titanium nitride film, for example, afilm satisfying a composition formula of AlTiN_(x) (x is a real numbergreater than 0 and less than or equal to 3) or a composition formula ofAlTi_(x)N_(y) (x is a real number greater than 0 and less than or equalto 2, y is a real number greater than 0 and less than or equal to 4) isfurther preferably used.

The temperature of the heat treatment is preferably as high as possiblebecause a reduction in the resistance of the third region isaccelerated. The temperature of the heat treatment is determined inconsideration of the heat resistance of the gate electrode, for example.The temperature can be set higher than or equal to 150° C. and lowerthan or equal to 500° C., preferably higher than or equal to 200° C. andlower than or equal to 450° C., further preferably higher than or equalto 250° C. and lower than or equal to 450° C., and still furtherpreferably higher than or equal to 300° C. and lower than or equal to400° C., for example. When the temperature of the heat treatment isapproximately 350° C., for example, semiconductor devices can bemanufactured at a high yield with production facilities using large-sizeglass substrates.

The heat treatment is performed in a state where the first layer isprovided in contact with the third region, whereby oxygen in the thirdregion is absorbed into the first layer, and thus, many oxygen vacanciescan be generated in the third region. Accordingly, the third regionhaving extremely low resistance can be formed. Meanwhile, since thefirst insulating layer is provided over the second region and thus thesecond region is not directly in contact with the first layer, oxygen isnot directly absorbed by the first layer. As a result, the third regionhaving extremely lower resistance than the second region can be formed.

The third region formed in such a manner has a feature in that itsresistance is not likely to be increased by subsequent process. There isno possibility that the conductivity of the third region is impaired byheat treatment in an atmosphere containing oxygen or by depositionprocess in an atmosphere containing oxygen, for example; thus, atransistor with favorable electrical characteristics and highreliability can be obtained.

When the first layer that has undergone the heat treatment hasconductivity, the first layer is preferably removed after the heattreatment. By contrast, when the first layer has insulating properties,the first layer can be left to function as a protective insulating film(second insulating layer).

It is particularly preferable to make the above-described aluminumnitride film or aluminum titanium nitride film remain because the filmhas excellent insulating properties.

More specific examples will be described below with reference todrawings.

Structure Example 1

FIG. 1(A) is a top view of a transistor 100, FIG. 1(B) corresponds to across-sectional view of a cut plane taken along a dashed-dotted lineA1-A2 in FIG. 1(A), and FIG. 1(C) corresponds to a cross-sectional viewof a cut plane taken along a dashed-dotted line B1-B2 in FIG. 1(A). Notethat in FIG. 1(A), some components of the transistor 100 (a protectiveinsulating layer and the like) are not illustrated. In addition, thedirection of the dashed-dotted line A1-A2 corresponds to a channellength direction, and the direction of the dashed-dotted line B1-B2corresponds to a channel width direction. Furthermore, some componentsare not illustrated in top views of transistors in the followingdrawings, as in FIG. 1(A).

The transistor 100 is provided over a substrate 102 and includes aninsulating layer 103, a semiconductor layer 108, an insulating layer110, a metal oxide layer 114, a conductive layer 112, an insulatinglayer 116, an insulating layer 118, and the like. The island-shapedsemiconductor layer 108 is provided over the insulating layer 103. Theinsulating layer 110 is provided to cover part of the top surface of theinsulating layer 103 and part of the top surface of the semiconductorlayer 108. The metal oxide layer 114 and the conductive layer 112 areprovided to be stacked in this order over the insulating layer 110 andeach include a portion overlapping with the semiconductor layer 108. Themetal oxide layer 114 and the conductive layer 112 are provided to belocated inward from an end portion of the insulating layer 110 in a planview. The insulating layer 116 is provided to cover the top surface anda side surface of the conductive layer 112, a side surface of the metaloxide layer 114, the top surface and a side surface of the insulatinglayer 110, the top surface and a side surface of the semiconductor layer108, and the top surface of the insulating layer 103. The insulatinglayer 118 is provided to cover the insulating layer 116.

Part of the conductive layer 112 functions as a gate electrode. Part ofthe insulating layer 110 functions as a gate insulating layer. Thetransistor 100 is what is called a top-gate transistor, in which thegate electrode is provided over the semiconductor layer 108.

In addition, as illustrated in FIGS. 1(A) and 1(B), the transistor 100may include a conductive layer 120 a and a conductive layer 120 b overthe insulating layer 118. The conductive layer 120 a and the conductivelayer 120 b function as a source electrode and a drain electrode. Theconductive layer 120 a and the conductive layer 120 b are electricallyconnected to regions 108N to be described later through an openingportion 141 a and an opening portion 141 b, respectively, which areprovided in the insulating layer 118 and the insulating layer 116.

The semiconductor layer 108 preferably contains a metal oxide.

The semiconductor layer 108 preferably contains indium, M (M is one kindor a plurality of kinds selected from gallium, aluminum, silicon, boron,yttrium, tin, copper, vanadium, beryllium, titanium, iron, nickel,germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, tungsten, and magnesium), and zinc, for example. It isparticularly preferable that M be one kind or a plurality of kindsselected from aluminum, gallium, yttrium, and tin.

It is particularly preferable to use an oxide containing indium,gallium, and zinc for the semiconductor layer 108.

The semiconductor layer 108 may have a stacked-layer structure in whichlayers with different compositions, layers with differentcrystallinities, or layers with different impurity concentrations arestacked.

The conductive layer 112 and the metal oxide layer 114 are processed tohave substantially the same top surface shapes.

Note that in this specification and the like, the expression “havingsubstantially the same top surface shapes” means that at least outlinesof stacked layers partly overlap with each other. For example, the caseof processing an upper layer and a lower layer with the use of the samemask pattern or mask patterns that are partly the same is included.However, in some cases, the outlines do not exactly overlap with eachother and the outline of the upper layer is located on an inner side ofthe outline of the lower layer or the outline of the upper layer islocated on an outer side of the outline of the lower layer; such a caseis also represented by the expression “having substantially the same topsurface shapes.”

The metal oxide layer 114 positioned between the insulating layer 110and the conductive layer 112 functions as a barrier film that preventsdiffusion of oxygen contained in the insulating layer 110 into aconductive layer 112 side. Furthermore, the metal oxide layer 114 alsofunctions as a barrier film that prevents diffusion of hydrogen andwater contained in the conductive layer 112 into an insulating layer 110side. For the metal oxide layer 114, a material that is less likely totransmit oxygen and hydrogen than at least the insulating layer 110 canbe used, for example.

Even in the case where a metal material that is likely to absorb oxygen,such as aluminum or copper, is used for the conductive layer 112, themetal oxide layer 114 can prevent diffusion of oxygen from theinsulating layer 110 into the conductive layer 112. Furthermore, even inthe case where the conductive layer 112 contains hydrogen, diffusion ofhydrogen from the conductive layer 112 to the semiconductor layer 108through the insulating layer 110 can be prevented. Consequently, carrierdensity of the semiconductor layer 108 in a channel formation region canbe extremely low.

For the metal oxide layer 114, an insulating material or a conductivematerial can be used. When the metal oxide layer 114 has insulatingproperties, the metal oxide layer 114 functions as part of the gateinsulating layer. By contrast, when the metal oxide layer 114 hasconductivity, the metal oxide layer 114 functions as part of the gateelectrode.

An insulating material having a higher permittivity than silicon oxideis preferably used for the metal oxide layer 114. It is particularlypreferable to use an aluminum oxide film, a hafnium oxide film, ahafnium aluminate film, or the like because drive voltage can bereduced.

For the metal oxide layer 114, a conductive oxide such as indium oxide,indium tin oxide (ITO), or indium tin oxide containing silicon (ITSO)can also be used, for example. A conductive oxide containing indium isparticularly preferable because of its high conductivity.

For the metal oxide layer 114, an oxide material containing one or moreof the same elements as those of the semiconductor layer 108 ispreferably used. It is particularly preferable to use an oxidesemiconductor material that can be used for the semiconductor layer 108.Here, a metal oxide film formed using the same sputtering target as thatfor the semiconductor layer 108 is preferably applied to the metal oxidelayer 114 because an apparatus can be shared.

Alternatively, when a metal oxide material containing indium and galliumis used for both the semiconductor layer 108 and the metal oxide layer114, a material whose composition (content ratio) of gallium is higherthan that in the semiconductor layer 108 is preferably used because anoxygen blocking property can be further increased. In that case, the useof a material whose composition of indium is higher than that in themetal oxide layer 114 for the semiconductor layer 108 enables anincrease in the field-effect mobility of the transistor 100.

The metal oxide layer 114 is preferably formed using a sputteringapparatus. In the case where an oxide film is formed using a sputteringapparatus, forming the oxide film in an atmosphere containing an oxygengas can suitably supply oxygen into the insulating layer 110 or thesemiconductor layer 108, for example.

The semiconductor layer 108 includes the channel formation region, whichoverlaps with the conductive layer 112 with the insulating layer 110therebetween. The semiconductor layer 108 also includes a pair ofregions 108L between which the channel formation region is sandwichedand a pair of regions 108N on outer sides of the regions 108L. Theregions 108L are each a region of the semiconductor layer 108 thatoverlaps with the insulating layer 110 and does not overlap with theconductive layer 112. The regions 108N are each a region of thesemiconductor layer 108 that overlaps with neither the conductive layer112 nor the insulating layer 110, and a region in contact with theinsulating layer 116.

Each of the region 108L and the region 108N can also be regarded as aregion having lower resistance than the channel formation region, aregion having higher carrier concentration than the channel formationregion, a region having a higher oxygen defect density than the channelformation region, a region having higher impurity concentration than thechannel formation region, or an n-type region. The region 108N can alsobe regarded as a region having lower resistance than the region 108L, aregion having higher carrier concentration than the region 108L, aregion having a higher oxygen defect density than the region 108L, aregion having higher impurity concentration than the region 108L, or ann-type region.

The region 108L and the region 108N are each a region containing animpurity element (first element). Examples of the impurity elementinclude hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur,arsenic, aluminum, magnesium, and a rare gas. Note that typical examplesof a rare gas include helium, neon, argon, krypton, and xenon. Inparticular, boron, phosphorus, magnesium, or aluminum is preferablycontained. Alternatively, two or more of these elements may becontained.

The region 108N may include a region having higher concentration of theabove impurity than the region 108L. The region 108N may have animpurity concentration peak at a deeper level than that of the region108L.

For each of the insulating layer 103 and the insulating layer 110 thatare in contact with the channel formation region of the semiconductorlayer 108, an oxide film is preferably used. For example, an oxide filmsuch as a silicon oxide film, a silicon oxynitride film, or an aluminumoxide film can be used. Accordingly, oxygen released from the insulatinglayer 103 and the insulating layer 110 can be supplied to the channelformation region of the semiconductor layer 108 by heat treatment or thelike in the manufacturing process of the transistor 100 to reduce oxygenvacancies in the semiconductor layer 108.

Part of the end portion of the insulating layer 110 is positioned overthe semiconductor layer 108. The insulating layer 110 includes a regionoverlapping with the conductive layer 112 and functioning as the gateinsulating layer and a portion not overlapping with the conductive layer112 (i.e., a portion overlapping with the region 108L).

The insulating layer 110 includes a region 110 d that contains theabove-described impurity element in a region not overlapping with theconductive layer 112. The region 110 d is positioned at least in thevicinity of an interface with the region 108L. Furthermore, it ispreferable that the region 110 d not be provided in a portion that is incontact with the channel formation region of the semiconductor layer108, as illustrated in FIGS. 1(B) and 1(C).

The insulating layer 103 includes, in a region not overlapping with theconductive layer 112, a region 103 d that contains the above-describedimpurity element in the vicinity of an interface with the insulatinglayer 110 and in the vicinity of an interface with the insulating layer116. Although not illustrated, the region 103 d may be provided also inthe vicinity of an interface with the region 108N or the region 108L. Inthat case, a portion overlapping with the region 108N or the region 108Lhas lower impurity concentration than a portion in contact with theinsulating layer 110 or the insulating layer 116.

Here, the region 108L preferably has a concentration gradient such thatthe impurity concentration is higher in a portion closer to theinsulating layer 110. In that case, the total amount of the impurityelement in the region 108L can be smaller than that in the case wherethe concentration is uniform throughout the entire region 108L; thus,the amount of the impurity that might be diffused into the channelformation region owing to the influence of heat applied during themanufacturing process or the like can be kept small.

The region 108N may have a similar concentration gradient. In that case,an upper portion of the region 108N has lower resistance, and thuscontact resistance with the conductive layer 120 a (or the conductivelayer 120 b) can be more effectively reduced.

In addition, the region 110 d preferably has a concentration gradientsuch that the impurity concentration is higher in a portion closer tothe semiconductor layer 108. In the insulating layer 110 to which anoxide film from which oxygen can be released by heating is applied,release of oxygen in the region 110 d to which the impurity element isadded can be inhibited as compared with that in the other regions. Thus,the region 110 d that is positioned in the vicinity of an interface withthe region 108L in the insulating layer 110 can function as a blockinglayer against oxygen and can effectively reduce oxygen supplied to theregion 108L.

As described later, treatment for adding the impurity element to theregion 108L, the region 108N, and the region 110 d can be performedusing the conductive layer 112 as a mask. Accordingly, the region 108L,the region 108N, and the region 110 d can be formed in a self-alignedmanner. Since the insulating layer 110 is provided over the region 108Lat this time, the region 108N and the region 108L may have differentimpurity concentration gradient profiles in the depth direction.

To show high-impurity-concentration portions exaggeratedly, FIGS. 1(B)and 1(C) and the like illustrate parts of the insulating layer 110 andthe insulating layer 103 with a hatch pattern as the region 110 d andthe region 103 d; however, the impurity element may be actuallycontained in the entire insulating layer 110 and insulating layer 103 ina thickness direction.

The region 108N, the region 108L, and the region 110 d each preferablyinclude a region whose impurity concentration is higher than or equal to1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²³ atoms/cm³, preferablyhigher than or equal to 5×10¹⁹ atoms/cm³ and lower than or equal to5×10²² atoms/cm³, and further preferably higher than or equal to 1×10²⁰atoms/cm³ and lower than or equal to 1×10²² atoms/cm³. In addition, theregion 108L preferably includes a portion having higher impurityconcentration than the region 110 d of the insulating layer 110 becausethe electrical resistance of the region 108L can be further effectivelyreduced. Furthermore, the region 108N preferably includes a portionhaving higher impurity concentration than the region 108L.

The concentrations of the impurities contained in the region 108L, theregion 108N, and the region 110 d can be analyzed by an analysis methodsuch as secondary ion mass spectrometry (SIMS) or X-ray photoelectronspectroscopy (XPS), for example. In the case of using XPS analysis, itis possible to find out concentration distribution in the depthdirection by the combination of XPS analysis and ion sputtering from afront surface side or a rear surface side.

In addition, the impurity element preferably exists in an oxidized statein the region 108L and the region 108N. For example, it is preferable touse an element that is easily oxidized, such as boron, phosphorus,magnesium, aluminum, or silicon, as the impurity element. Such anelement that is easily oxidized can exist stably in a state of beingbonded to oxygen in the semiconductor layer 108 to be oxidized and thuscan be inhibited from being released even when a high temperature (e.g.,higher than or equal to 400° C., higher than or equal to 600° C., orhigher than or equal to 800° C.) is applied in a later step.Furthermore, when the impurity element takes oxygen in the semiconductorlayer 108 away, many oxygen vacancies are generated in the region 108Land the region 108N. The oxygen vacancies are bonded to hydrogen in thefilm to serve as carrier supply sources; thus, the region 108L and theregion 108N are in an extremely low-resistance state.

Note that an increase in resistance might be caused if much oxygen issupplied to the region 108L and the region 108N from the outside or afilm near the region 108L and the region 108N at the time of performinghigh-temperature treatment in a later step. Thus, in the case wherehigh-temperature treatment is performed, the treatment is preferablyperformed with the semiconductor layer 108 covered with the insulatinglayer 116 that has a high barrier property against oxygen.

In addition, the impurity element preferably exists in an oxidized statealso in the region 110 d. Since such an element that is easily oxidizedcan exist stably in a state of being bonded to oxygen in the insulatinglayer 110 to be oxidized, the element can be inhibited from beingreleased even when a high temperature is applied in a later step.Furthermore, particularly in the case where oxygen (also referred to asexcess oxygen) that might be released by heating is contained in theinsulating layer 110, excess oxygen and the impurity element are bondedto each other and stabilized, so that oxygen can be inhibited from beingsupplied from the region 110 d to the region 108L. Moreover, oxygen isless likely to be diffused into the region 110 d containing the impurityelement in the oxidized state, so that oxygen can also be prevented frombeing supplied from a portion above the region 110 d to the region 108Lthrough the region 110 d.

For example, in the case where boron is used as the impurity element,boron contained in the region 108L, the region 108N, and the region 110d can exist in a state of being bonded to oxygen. This can be confirmedwhen a spectrum peak attributed to a B₂O₃ bond is observed in XPSanalysis. Furthermore, in XPS analysis, the intensity of a spectrum peakattributed to a state where a boron element exists alone is so low thatthe spectrum peak is not observed or is buried in background noise atthe measurement lower limit.

The insulating layer 116 is provided in contact with the region 108N ofthe semiconductor layer 108.

The insulating layer 116 functions as a hydrogen supply source to theregion 108N. The insulating layer 116 is preferably a film from whichhydrogen is released by heating, for example. Such an insulating layer116 is provided in contact with the region 108N and heat treatment isperformed after the formation of the insulating layer 116, wherebyhydrogen can be supplied to the region 108N, leading to a reduction inresistance.

The insulating layer 116 is preferably a film formed using a gascontaining a hydrogen element as a formation gas used in the formation.Accordingly, hydrogen can be effectively supplied to the region 108Nalso in the formation of the insulating layer 116.

For the insulating layer 116, an insulating film of silicon nitride,silicon nitride oxide, silicon oxynitride, aluminum nitride, aluminumnitride oxide, or the like can be used, for example.

The region 108N is in a state of containing many oxygen vacanciesbecause the impurity elements are added thereto as described above.Thus, hydrogen contained in the semiconductor layer 108 and hydrogensupplied from the insulating layer 116 can further increase the carrierdensity.

Meanwhile, since the region 108L is not in contact with the insulatinglayer 116 owing to the insulating layer 110 therebetween, the amount ofsupplied hydrogen is smaller than that of the region 108N. Furthermore,the region 108L has lower impurity concentration than the region 108Nand thus can be in a state of having higher resistance than the region108N.

Alternatively, an insulating film containing a nitride can be used asthe insulating layer 116 in contact with the region 108N. When theinsulating layer 116 containing a nitride is provided in contact withthe region 108N, an effect of further increasing the conductivity of theregion 108N is attained. Moreover, heat treatment is preferablyperformed in a state where the insulating layer 116 is in contact withthe region 108N because a reduction in resistance is accelerated.

As a nitride that can be used for the insulating layer 116, for example,a nitride of a semiconductor material, such as silicon nitride orgallium nitride, or a metal nitride such as aluminum nitride isparticularly preferably used. Silicon nitride, for example, has ablocking property against hydrogen and oxygen; thus, both diffusion ofhydrogen from the outside into the semiconductor layer and release ofoxygen from the semiconductor layer to the outside can be prevented,which leads to a highly reliable transistor.

In the case of using a metal nitride, it is preferable to use a nitrideof aluminum, titanium, tantalum, tungsten, chromium, or ruthenium. It isparticularly preferable that aluminum or titanium be contained. Forexample, an aluminum nitride film formed by a reactive sputtering methodusing aluminum as a sputtering target and a nitrogen-containing gas as adeposition gas can be a film having both an extremely high insulatingproperty and an extremely high blocking property against hydrogen andoxygen when the flow rate of a nitrogen gas with respect to the totalflow rate of the deposition gas is appropriately controlled. Thus, whensuch an insulating film containing a metal nitride is provided incontact with the semiconductor layer, the resistance of thesemiconductor layer can be reduced, diffusion of oxygen from thesemiconductor layer to the outside, and diffusion of hydrogen into thesemiconductor layer can be favorably prevented.

In the case where aluminum nitride is used as the metal nitride, thethickness of the insulating layer containing aluminum nitride ispreferably 5 nm or more. A film with such a small thickness can haveboth a high blocking property against hydrogen and oxygen and a functionof reducing the resistance of the semiconductor layer. Note that thereis no upper limit of the thickness of the insulating layer; however, thethickness is preferably less than or equal to 500 nm, further preferablyless than or equal to 200 nm, and still further preferably less than orequal to 50 nm in consideration of productivity.

In the case of using an aluminum nitride film as the insulating layer116, it is preferable to use a film that satisfies the compositionformula AlN_(x) (x is a real number greater than 0 and less than orequal to 2, and preferably, x is a real number greater than 0.5 and lessthan or equal to 1.5). In that case, a film having an excellentinsulating property and high thermal conductivity can be obtained, andthus dissipation of heat generated in driving the transistor 100 can beincreased.

Alternatively, an aluminum titanium nitride film, a titanium nitridefilm, or the like can be used as the insulating layer 116.

Such an insulating layer 116 is provided in contact with the region108N, whereby the insulating layer 116 absorbs oxygen in the region 108Nand oxygen vacancies can be formed in the region 108N. In the case wherea film containing a metal oxide is used as the insulating layer 116 inthat case, a layer containing an oxide of a metal element (e.g.,aluminum) contained in the insulating layer 116 is formed between theinsulating layer 116 and the region 108N, in some cases.

Here, in the case where a metal oxide film containing indium is used asthe semiconductor layer 108, a region where indium oxide is precipitatedor a region having a high indium concentration is sometimes formed inthe region 108N in the vicinity of the interface with the insulatinglayer 116. Such a region can sometimes be observed by an analysis methodsuch as X-ray photoelectron spectroscopy (XPS), for example.

The region 108N can be a region containing more oxygen vacancies thanthe region 108L as described above, and thus can be a region havinglower resistance than the region 108L. Moreover, when an insulating filmcontaining a metal oxide is used as the insulating layer 116, ahigh-conductivity region where indium oxide is deposited is formed inthe region 108N in the vicinity of the interface with the insulatinglayer 116, leading to a lower-resistance region.

The insulating layer 118 functions as a protective layer for protectingthe transistor 100. An inorganic insulating material such as an oxide ora nitride can be used for the insulating layer 118, for example. Morespecifically, for example, an inorganic insulating material such assilicon nitride, silicon nitride oxide, silicon oxynitride, aluminumoxide, aluminum oxynitride, aluminum nitride, hafnium oxide, or hafniumaluminate can be used. Furthermore, the insulating layer 118 can be usedas a planarization layer. In that case, an organic resin material can beused for the insulating layer 118.

Note that although the case where a stacked-layer structure of theinsulating layer 116 and the insulating layer 118 is employed as theprotective layer is described here, the insulating layer 118 isunnecessary if not needed. The insulating layer 118 may have astacked-layer structure of two or more layers.

Here, the semiconductor layer 108 and oxygen vacancies that might beformed in the semiconductor layer 108 will be described.

Oxygen vacancies formed in the channel formation region of thesemiconductor layer 108 adversely affect the transistor characteristicsand therefore cause a problem. For example, when an oxygen vacancy isformed in the semiconductor layer 108, the oxygen vacancy might bebonded to hydrogen to serve as a carrier supply source. The carriersupply source generated in the channel formation region causes a changein the electrical characteristics, typified by a shift in the thresholdvoltage, of the transistor 100. Therefore, it is preferable that theamount of oxygen vacancies in the channel formation region be as smallas possible.

In view of this, one embodiment of the present invention has a structurein which insulating films in the vicinity of the channel formationregion of the semiconductor layer 108, specifically, the insulatinglayer 110 positioned above the channel formation region and theinsulating layer 103 positioned below the channel formation region eachinclude an oxide film. When oxygen is moved from the insulating layer103 and the insulating layer 110 to the channel formation region by heatduring the manufacturing process or the like, the amount of oxygenvacancies in the channel formation region can be reduced.

In addition, the semiconductor layer 108 preferably includes a regionwhere the atomic ratio of In to M is greater than 1. A higher percentageof In content results in higher field-effect mobility of the transistor.

Here, in the case of a metal oxide containing In, Ga, and Zn, bondingstrength between In and oxygen is weaker than bonding strength betweenGa and oxygen; thus, with a higher percentage of In content, oxygenvacancies are likely to be generated in the metal oxide film. There is asimilar tendency even when a metal element shown above as M is usedinstead of Ga. The existence of a large amount of oxygen vacancies inthe metal oxide film leads to a reduction in electrical characteristicsand a reduction in reliability of the transistor.

However, in one embodiment of the present invention, an extremely largeamount of oxygen can be supplied into the channel formation region ofthe semiconductor layer 108 containing a metal oxide; thus, a metaloxide material with a high percentage of In content can be used.Accordingly, it is possible to achieve a transistor with extremely highfield-effect mobility, stable electrical characteristics, and highreliability.

For example, a metal oxide in which the atomic ratio of In to M is 1.5or higher, 2 or higher, 3 or higher, 3.5 or higher, or 4 or higher canbe suitably used.

It is particularly preferable that the atomic ratio of In, M, and Zn inthe semiconductor layer 108 be In:M:Zn=4:2:3 or in the neighborhoodthereof. Alternatively, the atomic ratio of In, M, and Zn is preferablyIn:M:Zn=5:1:6 or in the neighborhood thereof. Furthermore, as thecomposition of the semiconductor layer 108, the atomic proportions ofIn, M, and Zn in the semiconductor layer 108 may be approximately equalto each other. That is, a material in which the atomic ratio of In, M,and Zn is In:M:Zn=1:1:1 or in the neighborhood thereof may be included.

For example, with the use of the transistor with high field-effectmobility in a gate driver that generates a gate signal, a display devicewith small frame width (also referred to as a narrow frame) can beprovided. Furthermore, with the use of the transistor with highfield-effect mobility in a source driver (particularly a demultiplexerconnected to an output terminal of a shift register included in thesource driver), a display device to which fewer wirings are connectedcan be provided.

Note that even when the semiconductor layer 108 includes the regionwhere the atomic ratio of In to M is higher than 1, the field-effectmobility might be low if the semiconductor layer 108 has highcrystallinity. The crystallinity of the semiconductor layer 108 can beanalyzed by using X-ray diffraction (XRD) or a transmission electronmicroscope (TEM), for example.

Here, by reducing the impurity concentration and reducing the density ofdefect states (reducing oxygen vacancies) in the channel formationregion of the semiconductor layer 108, the carrier density in the filmcan be reduced. A transistor in which such a metal oxide film is usedfor a channel formation region of a semiconductor layer rarely haselectrical characteristics with a negative threshold voltage (alsoreferred to as normally-on). Furthermore, a transistor using such ametal oxide film can have characteristics of an extremely low off-statecurrent.

The use of a metal oxide film with high crystallinity for thesemiconductor layer 108 enables damage inhibition during the processingof the semiconductor layer 108 and the formation of the insulating layer110; thus, a highly reliable transistor can be obtained. Meanwhile, theuse of a metal oxide film with relatively low crystallinity for thesemiconductor layer 108 improves electric conductivity; thus, atransistor with high field-effect mobility can be obtained.

A metal oxide film having a CAAC (c-axis aligned crystal) structuredescribed later, a metal oxide film having an nc (nano crystal)structure, or a metal oxide film in which a CAAC structure and an ncstructure are mixed is preferably used as the semiconductor layer 108.

In addition, the semiconductor layer 108 may have a stacked-layerstructure of two or more layers.

For example, the semiconductor layer 108 in which two or more metaloxide films with different compositions are stacked can be used. Forinstance, in the case of using an In-M-Zn oxide, it is preferable to usea stack of two or more films each formed using a sputtering target withan atomic ratio of In:M:Zn=5:1:6, In:M:Zn=4:2:3, In:MZn=1:1:1,In:MZn=2:2:1, In:M:Zn=1:3:4, or In:M:Zn=1:3:2 or in the neighborhoodthereof.

Alternatively, the semiconductor layer 108 in which two or more metaloxide films with different crystallinities are stacked can be used. Inthat case, the metal oxide films are preferably successively formedwithout exposure to the air using the same oxide target under differentdeposition conditions.

In that case, a stacked-layer structure of a metal oxide film having annc structure and a metal oxide film having a CAAC structure can be usedfor the semiconductor layer 108. Alternatively, a stacked-layerstructure of a metal oxide film having an nc structure and a metal oxidefilm having an nc structure may be used. Note that for a function or amaterial composition of a metal oxide that can be suitably used for asemiconductor layer 108 a and a semiconductor layer 108 b, reference canbe made to the description of a CAC (Cloud-Aligned Composite) describedlater.

The oxygen flow rate ratio at the time of depositing an earlier-formedfirst metal oxide film is set lower than that at the time of depositinga subsequently formed second metal oxide film, for example.Alternatively, a condition without oxygen flowing is employed at thetime of depositing the first metal oxide film. In such a manner, oxygencan be effectively supplied at the time of depositing the second metaloxide film. In addition, the first metal oxide film can have lowercrystallinity and higher electrical conductivity than the second metaloxide film. Meanwhile, when the second metal oxide film provided in anupper portion has higher crystallinity than the first metal oxide film,damage caused at the time of processing the semiconductor layer 108 ordepositing the insulating layer 110 can be inhibited.

More specifically, the oxygen flow rate ratio at the time of depositingthe first metal oxide film is higher than or equal to 0% and lower than50%, preferably higher than or equal to 0% and lower than or equal to30%, further preferably higher than or equal to 0% and lower than orequal to 20%, typically 10%. In addition, the oxygen flow rate ratio atthe time of depositing the second metal oxide film is higher than orequal to 50% and lower than or equal to 100%, preferably higher than orequal to 60% and lower than or equal to 100%, further preferably higherthan or equal to 80% and lower than or equal to 100%, still furtherpreferably higher than or equal to 90% and lower than or equal to 100%,typically 100%. Furthermore, although the conditions at the time of thedeposition, such as pressure, temperature, and power may, vary betweenthe first metal oxide film and the second metal oxide film, it ispreferable to employ the same conditions except for the oxygen flow rateratio because the time required for deposition steps can be shortened.

With such a structure, the transistor 100 with excellent electricalcharacteristics and high reliability can be achieved.

Structure Example 2

A structure example of a transistor whose structure is partly differentfrom that of Structure Example 1 will be described below. Note thatdescription of the same portions as those in Structure Example 1 isomitted below in some cases. Furthermore, in drawings that are referredto later, the same hatching pattern is applied to portions havingfunctions similar to those in Structure Example 1, and the portions arenot denoted by reference numerals in some cases.

FIG. 2(A) is a top view of a transistor 100A. FIG. 2(B) is across-sectional view of the transistor 100A in a channel lengthdirection. FIG. 2(C) is a cross-sectional view of the transistor 100A ina channel width direction.

The transistor 100A is different from Structure Example 1 mainly inincluding a conductive layer 106 between the substrate 102 and theinsulating layer 103. The conductive layer 106 includes a regionoverlapping with the channel formation region and the region 108L of thesemiconductor layer 108 and the conductive layer 112.

In the transistor 100A, the conductive layer 106 has a function of afirst gate electrode (also referred to as a bottom gate electrode), andthe conductive layer 112 has a function of a second gate electrode (alsoreferred to as a top gate electrode). In addition, part of theinsulating layer 103 functions as a first gate insulating layer, andpart of the insulating layer 110 functions as a second gate insulatinglayer.

A portion of the semiconductor layer 108 that overlaps with at least oneof the conductive layer 112 and the conductive layer 106 functions as achannel formation region. Note that for easy explanation, a portion ofthe semiconductor layer 108 that overlaps with the conductive layer 112will be sometimes referred to as a channel formation region in thefollowing description; however, a channel can also be actually formed ina portion not overlapping with the conductive layer 112 and overlappingwith the conductive layer 106 (a portion including the region 108L andthe region 108N).

In addition, as illustrated in FIGS. 2(A) and 2(C), the conductive layer106 may be electrically connected to the conductive layer 112 through anopening portion 142 provided in the metal oxide layer 114, theinsulating layer 110, and the insulating layer 103. In that case, thesame potential can be supplied to the conductive layer 106 and theconductive layer 112.

For the conductive layer 106, a material similar to that for theconductive layer 112, the conductive layer 120 a, or the conductivelayer 120 b can be used. In particular, a material containing copper ispreferably used for the conductive layer 106 because wiring resistancecan be reduced. When a material containing a high-melting-point metal,such as tungsten or molybdenum, is used for the conductive layer 106,treatment in a later step can be performed at high temperatures.

In addition, as illustrated in FIGS. 2(A) and 2(C), the conductive layer112 and the conductive layer 106 preferably extend beyond an end portionof the semiconductor layer 108 in the channel width direction. In thatcase, as illustrated in FIG. 2(C), a structure is employed in which thesemiconductor layer 108 in the channel width direction is entirelycovered with the conductive layer 112 and the conductive layer 106 withthe insulating layer 110 and the insulating layer 103 therebetween.

With such a structure, the semiconductor layer 108 can be electricallysurrounded by electric fields generated by a pair of gate electrodes. Atthis time, it is particularly preferable that the same potential beapplied to the conductive layer 106 and the conductive layer 112. Inthat case, electric fields for inducing a channel can be effectivelyapplied to the semiconductor layer 108, so that the on-state current ofthe transistor 100A can be increased. Thus, the transistor 100A can alsobe miniaturized.

Note that a structure in which the conductive layer 112 and theconductive layer 106 are not connected to each other may be employed. Inthat case, a constant potential may be applied to one of the pair ofgate electrodes, and a signal for driving the transistor 100A may beapplied to the other of the pair of gate electrodes. In this case, thepotential applied to one of the electrodes can control the thresholdvoltage at the time of driving the transistor 100A with the otherelectrode.

The above is the description of Structure Example 2.

Manufacturing Method Example

A manufacturing method of the semiconductor device of one embodiment ofthe present invention will be described below with reference todrawings. Here, description will be made giving, as an example, thetransistor 100A illustrated in the above structure example.

Note that thin films that constitute the semiconductor device(insulating films, semiconductor films, conductive films, and the like)can be formed by a sputtering method, a chemical vapor deposition (CVD)method, a vacuum evaporation method, a pulsed laser deposition (PLD)method, an atomic layer deposition (ALD) method, or the like. Examplesof the CVD method include a plasma-enhanced chemical vapor deposition(PECVD: Plasma Enhanced CVD) method and a thermal CVD method. Inaddition, as an example of the thermal CVD method, a metal organicchemical vapor deposition (MOCVD: Metal Organic CVD) method can begiven.

Alternatively, the thin films (the insulating films, the semiconductorfilms, the conductive films, and the like) that constitute thesemiconductor device can be formed by a method such as spin coating,dipping, spray coating, ink-jetting, dispensing, screen printing, offsetprinting, a doctor knife method, slit coating, roll coating, curtaincoating, or knife coating.

In addition, when the thin films that constitute the semiconductordevice are processed, a photolithography method or the like can be usedfor the processing. Alternatively, the thin films may be processed by ananoimprinting method, a sandblasting method, a lift-off method, or thelike. Alternatively, island-shaped thin films may be directly formed bya deposition method using a blocking mask such as a metal mask.

There are the following two typical ways of a photolithography method.One is a method in which a resist mask is formed over a thin film thatis to be processed, the thin film is processed by etching or the like,and the resist mask is removed. The other is a method in which, after aphotosensitive thin film is deposited, exposure and development areperformed to process the thin film into a desired shape.

As light for exposure in a photolithography method, for example, ani-line (with a wavelength of 365 nm), a g-line (with a wavelength of 436nm), an h-line (with a wavelength of 405 nm), or light in which thei-line, the g-line, and the h-line are mixed can be used. Alternatively,ultraviolet light, KrF laser light, ArF laser light, or the like can beused. In addition, exposure may be performed by liquid immersionexposure technique. Furthermore, as the light for the exposure, extremeultra-violet (EUV: Extreme Ultra-violet) light or X-rays may be used.Moreover, instead of the light for the exposure, an electron beam canalso be used. It is preferable to use extreme ultra-violet light,X-rays, or an electron beam because extremely minute processing can beperformed. Note that in the case of performing exposure by scanning of abeam such as an electron beam, a photomask is unnecessary.

For etching of the thin films, a dry etching method, a wet etchingmethod, a sandblasting method, or the like can be used.

In each of drawings of FIG. 3 to FIG. 6 , cross sections of thetransistor 100A in the channel length direction and in the channel widthdirection in each step in the manufacturing process are illustratedtogether.

<Formation of Conductive Layer 106>

A conductive film is deposited over the substrate 102 and processed byetching to form the conductive layer 106 functioning as the first gateelectrode.

<Formation of Insulating Layer 103>

Then, the insulating layer 103 is formed to cover the substrate 102 andthe conductive layer 106 (FIG. 3(A)). The insulating layer 103 can beformed by a PECVD method, an ALD method, a sputtering method, or thelike.

After the insulating layer 103 is formed, treatment for supplying oxygento the insulating layer 103 may be performed. For example, plasmatreatment, heat treatment, or the like in an oxygen atmosphere can beperformed. Alternatively, oxygen may be supplied to the insulating layer103 by a plasma ion doping method, an ion implantation method, or thelike.

<Formation of Semiconductor Layer 108>

Next, a metal oxide film is deposited over the insulating layer 103 andprocessed to form the island-shaped semiconductor layer 108 (FIG. 3(B)).

The metal oxide film is preferably formed by a sputtering method using ametal oxide target.

In addition, an oxygen gas and an inert gas (such as a helium gas, anargon gas, or a xenon gas) may be mixed in depositing the metal oxidefilm. Note that when the proportion of the oxygen gas in the wholedeposition gas (hereinafter also referred to as an oxygen flow rateratio) at the time of depositing the metal oxide film is higher, thecrystallinity of the metal oxide film can be higher and a transistorwith high reliability can be achieved. By contrast, when the oxygen flowrate ratio is lower, the crystallinity of the metal oxide film is lowerand a transistor with a high on-state current can be obtained.

In the case of using a stacked-layer structure for the semiconductorlayer 108, it is preferable that successive deposition be performed inthe same deposition chamber using the same sputtering target because theinterface can be a favorable one. Although the deposition conditions,such as pressure, temperature, and power at the time of the depositionmay be different between the metal oxide films, it is particularlypreferable to employ the same conditions except for the oxygen flow rateratio because the time required for deposition steps can be shortened.In the case where metal oxide films having different compositions arestacked, it is preferable to perform successive deposition withoutexposure to the air.

The deposition conditions of the metal oxide film are preferably setsuch that a metal oxide film having a CAAC structure, a metal oxide filmhaving an nc structure, or a metal oxide film in which a CAAC structureand an nc structure are mixed is obtained. Note that the depositionconditions for the metal oxide film formed to have a CAAC structure andthe deposition conditions for the metal oxide film formed to have an ncstructure each vary depending on the composition of a sputtering targetto be used; thus, pressure, power, and the like, in addition to asubstrate temperature and an oxygen flow rate ratio, are set asappropriate depending on the composition.

In addition, as the deposition conditions of the metal oxide film, thesubstrate temperature is set higher than or equal to room temperatureand lower than or equal to 450° C., and the substrate temperature ispreferably set higher than or equal to room temperature and lower thanor equal to 300° C., further preferably higher than or equal to roomtemperature and lower than or equal to 200° C., and still furtherpreferably higher than or equal to room temperature and lower than orequal to 140° C. The substrate temperature is preferably set higher thanor equal to room temperature and lower than 140° C. in the case where alarge-sized glass substrate or a resin substrate is used as thesubstrate 102, for example, because high productivity is achieved.Furthermore, when the metal oxide film is deposited with the substratetemperature set at room temperature or without intentional heating, thecrystallinity can be made low.

In addition, it is preferable to perform treatment for desorbing water,hydrogen, an organic substance, or the like adsorbed onto a surface ofthe insulating layer 103 or treatment for supplying oxygen into theinsulating layer 103 before deposition of the metal oxide film. Forexample, heat treatment can be performed at a temperature higher than orequal to 70° C. and lower than or equal to 200° C. in a reduced-pressureatmosphere. Alternatively, plasma treatment may be performed in anoxygen-containing atmosphere. In addition, when plasma treatment isperformed in an atmosphere containing a nitric oxide gas, an organicsubstance on the surface of the insulating layer 103 can be suitablyremoved. After such treatment, the metal oxide film is preferablydeposited successively without exposure of the surface of the insulatinglayer 103 to the air.

For processing of the metal oxide film, either one or both a wet etchingmethod and a dry etching method is used. At this time, part of theinsulating layer 103 that does not overlap with the semiconductor layer108 is etched and thinned in some cases.

In addition, after the metal oxide film is deposited or processed intothe semiconductor layer 108, heat treatment may be performed to removehydrogen or water in the metal oxide film or the semiconductor layer108. The temperature of the heat treatment can be typically higher thanor equal to 150° C. and lower than the strain point of the substrate,higher than or equal to 250° C. and lower than or equal to 450° C., orhigher than or equal to 300° C. and lower than or equal to 450° C.

The heat treatment can be performed in an atmosphere containing a raregas or nitrogen. Alternatively, heating may be performed in theatmosphere, and then heating may be performed in an oxygen-containingatmosphere. Note that it is preferable that the atmosphere of the heattreatment not contain hydrogen, water, or the like. An electric furnace,an RTA apparatus, or the like can be used for the heat treatment. Theuse of an RTA apparatus can shorten the heat treatment time.

<Formation of Insulating Film 110 f and Metal Oxide Film 114 f>

Next, an insulating film 110 f and a metal oxide film 114 f are formedso as to cover the insulating layer 103 and the semiconductor layer 108.

The insulating film 110 f is a film that is to be the insulating layer110 later. As the insulating film 110 f, for example, an oxide film suchas a silicon oxide film or a silicon oxynitride film is preferablyformed using a plasma-enhanced chemical vapor deposition apparatus (aPECVD apparatus or simply referred to as a plasma CVD apparatus).Alternatively, a PECVD method using a microwave may be employed.

The metal oxide film 114 f is a film that is to be the metal oxide layer114 later. The metal oxide film 114 f is preferably formed by asputtering method in an oxygen-containing atmosphere, for example. Inthat case, oxygen can be supplied to the insulating film 110 f at thetime of deposition of the metal oxide film 114 f.

In the case where the metal oxide film 114 f is formed by a sputteringmethod using an oxide target containing a metal oxide similar to that inthe case of the semiconductor layer 108, the above method can bereferred to.

The metal oxide film 114 f may be formed by a reactive sputtering methodwith a metal target using oxygen as a deposition gas. When aluminum isused for the metal target, an aluminum oxide film can be deposited.

At the time of depositing the metal oxide film 114 f, the amount ofoxygen supplied into the insulating film 110 f can be increased with ahigher proportion of the oxygen flow rate to the total flow rate of thedeposition gas introduced into a deposition chamber of a depositionapparatus (a higher oxygen flow rate ratio) or with higher oxygenpartial pressure in the deposition chamber. The oxygen flow rate ratioor the oxygen partial pressure is, for example, higher than or equal to50% and lower than or equal to 100%, preferably higher than or equal to65% and lower than or equal to 100%, further preferably higher than orequal to 80% and lower than or equal to 100%, and still furtherpreferably higher than or equal to 90% and lower than or equal to 100%.It is particularly preferable that the oxygen flow rate ratio be 100%and the oxygen partial pressure be as close to 100% as possible.

When the metal oxide film 114 f is formed by a sputtering method in anoxygen-containing atmosphere in this manner, oxygen can be supplied tothe insulating film 110 f and release of oxygen from the insulating film110 f can be prevented during the deposition of the metal oxide film 114f As a result, an extremely large amount of oxygen can be enclosed inthe insulating film 110 f. Moreover, a large amount of oxygen can besupplied to the channel formation region of the semiconductor layer 108by heat treatment performed later, leading to a reduction in oxygenvacancies in the channel formation region; thus, a highly reliabletransistor can be achieved.

In addition, oxygen may be supplied from the insulating film 110 f tothe semiconductor layer 108 by heat treatment performed after theformation of the metal oxide film 114 f The heat treatment can beperformed at a temperature higher than or equal to 200° C. and lowerthan or equal to 400° C. in an atmosphere containing one or more ofnitrogen, oxygen, and a rare gas.

Then, the metal oxide film 114 f, the insulating film 110 f, and theinsulating layer 103 are partly etched to form an opening reaching theconductive layer 106. Accordingly, the conductive layer 112 to be formedlater can be electrically connected to the conductive layer 106 throughthe opening.

<Formation of Conductive Film 112 f>

Next, a conductive film 112 f to be the conductive layer 112 isdeposited over the metal oxide film 114 f (FIG. 3(C)). The conductivefilm 112 f is preferably deposited by a sputtering method using asputtering target of a metal or an alloy.

<Formation of Insulating Layer 110, Metal Oxide Layer 114, andConductive Layer 112>

Then, a resist mask 115 is formed over the conductive film 112 f Afterthat, the conductive film 112 f, the metal oxide film 114 f, and theinsulating film 110 f in a region not covered with the resist mask 115are etched by an anisotropic etching method (FIG. 4(A)).

The etching of the conductive film 112 f, the metal oxide film 114 f,and the insulating film 110 f may be performed at the same time underthe same etching conditions, or may be performed at least twice usingdifferent etching conditions or methods. For example, when theconductive film 112 f and the metal oxide film 114 f are etched firstand then the insulating film 110 f is etched under different etchingconditions, etching damage to the semiconductor layer 108 can bereduced.

Here, the resist mask 115 is preferably formed over a region to be theinsulating layer 110.

Next, side surfaces of the conductive film 112 f and the metal oxidefilm 114 f are etched using an isotropic etching method so that endfaces recede (also referred to as side etching). Consequently, theconductive layer 112 and the metal oxide layer 114 whose end portionsare located inward from the end portion of the insulating layer 110 in aplan view can be formed (FIG. 4(B)).

Here, it is preferable to select conditions or a method with which thesemiconductor layer 108 and the insulating layer 103 are etched aslittle as possible for the etching of the conductive film 112 f and themetal oxide film 114 f Although the side etching of the conductive film112 f and the metal oxide film 114 f may be performed in a state wherethe resist mask 115 is removed, it is preferable to make the resist mask115 remain because only the side surface of the conductive layer 112 canbe etched without reducing the thickness.

Note that although a method is described here in which side etching isperformed on the metal oxide film 114 f as well as the conductive film112 f, only the conductive film 112 f may be side-etched. In that case,the metal oxide layer 114 includes a region not overlapping with theconductive layer 112, like the insulating layer 110.

After the conductive layer 112 and the metal oxide layer 114 are formed,the resist mask 115 is removed.

<Treatment for Supplying Impurity Element (Formation of Region 108L)>

Next, treatment for supplying (also referred to as “adding” or“implanting”) an impurity element 140 to the insulating layer 110 andthe semiconductor layer 108 is performed using the conductive layer 112as a mask to form the region 108L, the region 110 d, and the region 103d (FIG. 5(A)). At this time, in the semiconductor layer 108 and theinsulating layer 110, the regions overlapping with the conductive layer112 are not supplied with the impurity element 140 owing to theconductive layer 112 serving as the mask.

A plasma ion doping method or an ion implantation method can be suitablyused for the supply of the impurity element 140. In these methods, aconcentration profile in a depth direction can be controlled with highaccuracy by the acceleration voltage and dosage of ions, or the like.The use of a plasma ion doping method can increase productivity. Inaddition, the use of an ion implantation method with mass separation canincrease the purity of an impurity element to be supplied.

In the treatment for supplying the impurity element 140, treatmentconditions are preferably controlled such that the concentration is thehighest at an interface between the semiconductor layer 108 and theinsulating layer 110, a portion in the semiconductor layer 108 near theinterface, or a portion in the insulating layer 110 near the interface.Accordingly, the impurity element 140 at an optimal concentration can besupplied to both the semiconductor layer 108 and the insulating layer110 in one treatment.

Examples of the impurity element 140 include hydrogen, boron, carbon,nitrogen, fluorine, phosphorus, sulfur, arsenic, aluminum, magnesium,silicon, and a rare gas. Note that typical examples of a rare gasinclude helium, neon, argon, krypton, and xenon. It is particularlypreferable to use boron, phosphorus, aluminum, magnesium, or silicon.

As a source gas of the impurity element 140, a gas containing theimpurity element can be used. In the case where boron is supplied,typically, a B₂H₆ gas, a BF₃ gas, or the like can be used. In addition,in the case where phosphorus is supplied, typically, a PH₃ gas can beused. Alternatively, a mixed gas in which these source gases are dilutedwith a rare gas may be used.

Alternatively, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, H₂,(C₅H₅)₂Mg, a rare gas, or the like can be used as the source gas. Inaddition, an ion source is not limited to a gas, and a solid or a liquidthat is vaporized by heating may be used.

Addition of the impurity element 140 can be controlled by setting theconditions such as the acceleration voltage and the dosage inconsideration of the compositions, densities, thicknesses, and the likeof the insulating layer 110 and the semiconductor layer 108.

For example, in the case where boron is added by an ion implantationmethod or a plasma ion doping method, the acceleration voltage can be,for example, within the range from 5 kV to 100 kV, preferably from 7 kVto 70 kV, and further preferably from 10 kV to 50 kV. In addition, thedosage can be, for example, within the range from 1×10¹³ ions/cm² to1×10¹⁷ ions/cm², preferably from 1×10¹⁴ ions/cm² to 5×10¹⁶ ions/cm², andfurther preferably from 1×10¹⁵ ions/cm² to 3×10¹⁶ ions/cm².

In addition, in the case where phosphorus is added by an ionimplantation method or a plasma ion doping method, the accelerationvoltage can be, for example, within the range from 10 kV to 100 kV,preferably from 30 kV to 90 kV, and further preferably from 40 kV to 80kV. Furthermore, the dosage can be, for example, within the range from1×10¹³ ions/cm² to 1×10¹⁷ ions/cm², preferably from 1×10¹⁴ ions/cm² to5×10¹⁶ ions/cm², and further preferably from 1×10¹⁵ ions/cm² to 3×10¹⁶ions/cm².

Note that a method for supplying the impurity element 140 is not limitedthereto; plasma treatment, treatment using thermal diffusion by heating,or the like may be used, for example. In plasma treatment, plasma isgenerated in a gas atmosphere containing an impurity element to be addedand plasma treatment is performed, so that the impurity element can beadded. A dry etching apparatus, an ashing apparatus, a plasma CVDapparatus, a high-density plasma CVD apparatus, or the like can be usedas an apparatus for generating the plasma.

Here, the impurity element 140 is added to a region not covered with theinsulating layer 110 (a region which is to be the region 108N later) atthe same time when the impurity element 140 is added to the region 108Lthrough the insulating layer 110. The concentration of the impurityelement 140 might be higher in this region than in the region 108L.Furthermore, the concentration gradient profile of the impurity element140 in the depth direction might be different from that of the region108L.

<Formation of Insulating Layer 116 and Region 108N>

Subsequently, treatment for supplying hydrogen into an exposed region ofthe semiconductor layer 108 is performed. Here, the insulating layer 116containing hydrogen is deposited in contact with the exposed region ofthe semiconductor layer 108 to supply hydrogen (FIG. 5(B)).

The insulating layer 116 is preferably formed by a plasma CVD methodusing a deposition gas containing hydrogen. For example, a siliconnitride film is deposited using a deposition gas containing a silane gasand an ammonia gas. The use of an ammonia gas in addition to a silanegas enables the film to contain a large amount of hydrogen. Furthermore,hydrogen can be supplied to the exposed portion of the semiconductorlayer 108 also at the time of deposition.

It is preferable to perform heat treatment after the deposition of theinsulating layer 116 so that part of hydrogen released from theinsulating layer 116 is supplied to part of the semiconductor layer 108.It is preferable that the heat treatment be performed at a temperaturehigher than or equal to 150° C. and lower than or equal to 450° C.,preferably higher than or equal to 200° C. and lower than or equal to400° C., in an atmosphere containing one or more of nitrogen, oxygen,and a rare gas.

Hydrogen is supplied in such a manner, whereby the region 108N havingextremely low resistance can be formed in the semiconductor layer 108.The region 108N can also be referred to as a region having highercarrier concentration, a region having a larger amount of oxygenvacancies, a region having higher hydrogen concentration, or a regionhaving higher impurity concentration than the region 108L.

Alternatively, the insulating layer 116 and the region 108N may beformed by the following method.

First, the insulating layer 116 is formed in contact with the exposedregion of the semiconductor layer 108.

For the insulating layer 116, a film containing at least one of metalelements such as aluminum, titanium, tantalum, tungsten, chromium, andruthenium is deposited. It is particularly preferable to contain atleast one of aluminum, titanium, tantalum, and tungsten. In particular,a nitride containing at least one of these metal elements or an oxidecontaining at least one of these metal elements can be suitably used. Anitride film such as an aluminum titanium nitride film, a titaniumnitride film, or an aluminum nitride film, or an oxide film such as analuminum titanium oxide film can be suitably used for the insulatinglayer 116.

Here, the insulating layer 116 is preferably formed by a sputteringmethod using a nitrogen gas or an oxygen gas as a deposition gas. Thus,the film quality can be easily controlled by controlling the flow rateof the deposition gas.

Subsequently, heat treatment is performed. By the heat treatment, theresistance of a region of the semiconductor layer 108 that are incontact with the insulating layer 116 is reduced, and the region 108Nhaving low resistance is formed in the semiconductor layer 108.

The heat treatment is preferably performed in an inert gas atmospheresuch as nitrogen or a rare gas. The temperature of the heat treatment ispreferably as high as possible and can be set in consideration of theheat resistance of the substrate 102, the conductive layer 106, theconductive layer 112, and the like. The temperature can be set higherthan or equal to 120° C. and lower than or equal to 500° C., preferablyhigher than or equal to 150° C. and lower than or equal to 450° C.,further preferably higher than or equal to 200° C. and lower than orequal to 400° C., and still further preferably higher than or equal to250° C. and lower than or equal to 400° C., for example. When thetemperature of the heat treatment is approximately 350° C., for example,the semiconductor device can be manufactured at a high yield withproduction facilities using a large-size glass substrate.

Note that since the insulating layer 116 is not removed here, the heattreatment can be performed at any step after the formation of theinsulating layer 116. The heat treatment may double as another heattreatment or a heating step.

By the heat treatment, oxygen in the semiconductor layer 108 isextracted to the insulating layer 116; thus, an oxygen vacancy isgenerated. When the oxygen vacancy is bonded to hydrogen in thesemiconductor layer 108, the carrier concentration is increased, and theresistance of the region 108N in contact with the insulating layer 116is reduced.

Alternatively, the metal element contained in the insulating layer 116may diffused into the semiconductor layer 108 by the heat treatment,whereby the semiconductor layer 108 is partly alloyed and reduced inresistance in some cases.

Alternatively, nitrogen and hydrogen contained in the insulating layer116 or nitrogen or the like contained in the atmosphere for the heattreatment may be diffused into the semiconductor layer 108 by the heattreatment, whereby the resistance of the region 108N in contact with theinsulating layer 116 is reduced in some cases.

The region 108N of the semiconductor layer 108, whose resistance hasbeen reduced because of the above complex action, becomes a highlystable low-resistance region. The region 108N formed in such a manner ischaracterized in that the resistance is not easily increased even iftreatment for supplying oxygen is performed in a later step, forexample.

Note that although an example is described here in which the insulatinglayer 116 having an insulating property is used as a layer used forforming the region 108N, the region 108N may be formed by forming a filmhaving conductivity in contact with a region to be the region 108N. Inthat case, it is preferable that the film having conductivity beinsulated by oxidization or nitridation after the formation of theregion 108N to obtain the insulating layer 116. Alternatively, the filmmay be removed after the formation of the region 108N so that astructure in which the insulating layer 116 is not provided is obtained.

In the above manner, the region 108N can be formed.

When the above-described heat treatment is performed, the region 108Land the region 108N can be in a low-resistance state more stably. Forexample, by the heat treatment at the above temperature, the impurityelement 140 is diffused moderately and the concentration is homogenizedlocally, so that the region 108L, the region 108N, and the region 110 deach having an ideal concentration gradient of the impurity element canbe formed. Note that when the temperature of the heat treatment is toohigh (e.g., higher than or equal to 500° C.), the impurity element 140is also diffused from the region 108L into the channel formation region,so that the electrical characteristics or reliability of the transistormight be degraded.

In addition, when the impurity element 140 is supplied to the region108L and the region 108N, defects generated in the semiconductor layer108 and the insulating layer 110 can be repaired by the heat treatmentin some cases.

Furthermore, oxygen can be supplied from the insulating layer 110 to thechannel formation region of the semiconductor layer 108 by the heattreatment. In that case, the region 110 d supplied with the impurityelement 140 is formed in a protruding portion of the insulating layer110 in the vicinity of an interface between the insulating layer 110 andthe region 108L; thus, oxygen released from the insulating layer 110 isinhibited from being diffused into the region 108L. As a result, theresistance of the region 108L can be effectively prevented from beingincreased. Moreover, in this case, the region 110 d is not formed in aregion of the insulating layer 110 that overlaps with the channelformation region of the semiconductor layer 108; thus, oxygen releasedfrom the insulating layer 110 can be selectively supplied to the channelformation region.

<Formation of Insulating Layer 118>

Next, the insulating layer 118 is formed over the insulating layer 116(FIG. 6(A)).

In the case where the insulating layer 118 is formed by a plasma CVDmethod at a too high deposition temperature, impurities contained in theregion 108N, the region 108L, and the like might be diffused into aperipheral portion including the channel formation region of thesemiconductor layer 108, depending on the impurities. As a result, theresistance of the channel formation region might be reduced or theelectrical resistance of the region 108N or the region 108L might beincreased. The deposition temperature of the insulating layer 116 or theinsulating layer 118 is preferably higher than or equal to 150° C. andlower than or equal to 400° C., further preferably higher than or equalto 180° C. and lower than or equal to 360° C., and still furtherpreferably higher than or equal to 200° C. and lower than or equal to250° C., for example. Deposition of the insulating layer 118 at lowtemperatures enables the transistor to have favorable electricalcharacteristics even when the transistor has a short channel length.

Furthermore, heat treatment may be performed after the formation of theinsulating layer 118.

<Formation of Opening Portion 141 a and Opening Portion 141 b>

Next, a mask is formed by lithography in a desired position on theinsulating layer 118, and then the insulating layer 118 and theinsulating layer 116 are partly etched to form an opening portion 141 aand an opening portion 141 b reaching the regions 108N.

<Formation of Conductive Layer 120 a and Conductive Layer 120 b>

Next, a conductive film is deposited over the insulating layer 118 tocover the opening portion 141 a and the opening portion 141 b, and theconductive film is processed into a desired shape, so that theconductive layer 120 a and the conductive layer 120 b are formed (FIG.6(B)).

Through the above process, the transistor 100A can be manufactured.

Modification Example of Manufacturing Method Example

A manufacturing method example that is partly different from the abovemanufacturing method example will be described below. Note thatdescription of the portions overlapping with the above is omitted anddifferent portions will be described.

Modification Example 1

First, the conductive layer 106, the insulating layer 103, and thesemiconductor layer 108 are formed in a manner similar to the above.Next, the insulating film 110 f, the metal oxide film 114 f, and theconductive film 112 f are formed, and the resist mask 115 is formed overthe conductive film 112 f.

Then, the metal oxide film 114 f and the conductive film 112 f areetched to form the metal oxide layer 114 and the conductive layer 112(FIG. 7(A)).

At this time, the metal oxide layer 114 and the conductive layer 112 areprocessed such that the end portions are located inward from the outlineof the resist mask 115. To form the metal oxide layer 114 and theconductive layer 112, for example, the metal oxide film 114 f and theconductive film 112 f are etched by anisotropic etching first and thenside etching is performed by isotropic etching.

Next, the insulating film 110 f is etched by anisotropic etching usingthe resist mask 115 to form the insulating layer 110 (FIG. 7(B)).Accordingly, the insulating layer 110 including a portion projectedbeyond the conductive layer 112 can be formed. After that, the resistmask 115 is removed.

Since the semiconductor layer 108 is not exposed at the time of theetching of the metal oxide film 114 f and the conductive film 112 f insuch a method, dispersal and attachment of their components on thesurface of the semiconductor layer 108 can be prevented. As a result, ahighly reliable transistor can be obtained.

Note that the above description can be referred to for the subsequentsteps.

Modification Example 2

First, the conductive layer 106, the insulating layer 103, and thesemiconductor layer 108 are formed in a manner similar to the above.Next, the insulating film 110 f, the metal oxide film 114 f, and theconductive film 112 f are formed. After that, a resist mask is formedover the conductive film 112 f and the conductive film 112 f and themetal oxide film 114 f are etched, so that the metal oxide layer 114 andthe conductive layer 112 are formed. The resist mask that is used atthis time may have the same pattern as the resist mask 115; a patternthat covers a region to be the conductive layer 112 is preferably used.Note that in the case of using the same pattern as that of the resistmask 115, the metal oxide film 114 f and the conductive film 112 f areside-etched as in Modification example 1. After that, the resist mask isremoved.

Next, the impurity element 140 is added to the insulating film 110 f andthe semiconductor layer 108 using the conductive layer 112 as a mask(FIG. 8(A)). Accordingly, the region 108L to which the impurity elementis added can be formed in a portion of the semiconductor layer 108 whichdoes not overlap with the conductive layer 112. At the same time, theregion 110 d to which the impurity element is added is formed in theinsulating film 110 f, and the region 103 d to which the impurityelement is added is formed in the insulating layer 103.

Then, the resist mask 115 is formed to cover the top surfaces and sidesurfaces of the conductive layer 112 and the metal oxide layer 114. Theresist mask 115 has a pattern that covers a region to be the insulatinglayer 110. After that, the insulating film 110 f is etched to form theinsulating layer 110 including a portion projected beyond the conductivelayer 112, and at the same time, part of the semiconductor layer 108 (aportion to be the region 108N) is exposed (FIG. 8(B)).

Note that hydrogen may be added to the exposed part of the semiconductorlayer 108 at this stage. For example, hydrogen may be added by a methodsuch as an ion doping method or an ion implantation method using theresist mask 115 as a mask. Alternatively, heat treatment may beperformed in an atmosphere containing hydrogen. In the case ofperforming heat treatment, the quality of the resist mask 115 might bechanged; thus, the heat treatment is preferably performed after theresist mask 115 is removed. Heat treatment performed in an atmospherecontaining hydrogen in a state where the conductive layer 112 is exposedcan expectedly produce a secondary effect of reducing and removing anoxide on the surface of the conductive layer 112.

Subsequently, the insulating layer 116 is formed in contact with theexposed portion of the semiconductor layer 108 and heat treatment isperformed in a manner similar to the above, whereby the region 108N isformed (FIG. 8(C)).

Since a region to be the region 108L and a region to be the region 108Nin the semiconductor layer 108 are both in a state of being covered withthe insulating film 110 f at the time of the addition of the impurityelement 140 in such a method, the impurity concentration profiles in thedepth direction can be substantially the same, leading to theimprovement in controllability. Furthermore, since the conductive layer112 and the metal oxide layer 114 are covered with the resist mask 115and not exposed at the time of the etching of the insulating film 110 f,some components of the conductive layer 112 and the metal oxide layer114 can be prevented from being attached to the surface of thesemiconductor layer 108.

Note that the above description can be referred to for the subsequentsteps.

The above is the description of the modification example of themanufacturing method example.

[Components of Semiconductor Device]

Next, components of the semiconductor device in this embodiment will bedescribed in detail.

<Substrate>

Although there is no particular limitation on a material and the like ofthe substrate 102, it is necessary that the substrate have heatresistance high enough to withstand at least heat treatment performedlater. For example, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate including silicon or siliconcarbide as a material, a compound semiconductor substrate of silicongermanium or the like, an SOT substrate, a glass substrate, a ceramicsubstrate, a quartz substrate, a sapphire substrate, or the like may beused as the substrate 102. Alternatively, these substrates provided withsemiconductor elements may be used as the substrate 102.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor 100 or the like may be formed directly on theflexible substrate. Alternatively, a separation layer may be providedbetween the substrate 102 and the transistor 100 or the like. Theseparation layer can be used in separating a semiconductor device fromthe substrate 102 after partly or wholly completing the semiconductordevice over the separation layer, and in transferring the separatedsemiconductor device to another substrate. In such a case, thetransistor 100 or the like can be transferred onto a substrate havinglow heat resistance or a flexible substrate as well.

<Insulating Layer 103>

The insulating layer 103 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method,or the like as appropriate. In addition, for example, the insulatinglayer 103 can be formed to have a single layer or stacked layer of anoxide insulating film or a nitride insulating film. Note that to improvethe properties of the interface with the semiconductor layer 108, atleast a region in the insulating layer 103 that is in contact with thesemiconductor layer 108 is preferably formed using an oxide insulatingfilm. Furthermore, a film from which oxygen is released by heating ispreferably used for the insulating layer 103.

For example, the insulating layer 103 can be provided to have a singlelayer or stacked layers using silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, hafnium oxide, galliumoxide, a Ga—Zn oxide, or the like.

In addition, in the case where a film other than an oxide film, such asa silicon nitride film, is used for a side of the insulating layer 103that is in contact with the semiconductor layer 108, pretreatment suchas oxygen plasma treatment is preferably performed on a surface incontact with the semiconductor layer 108 to oxidize the surface or thevicinity of the surface.

<Conductive Film>

The conductive layer 112 and the conductive layer 106 that function asthe gate electrodes, the conductive layer 120 a that functions as one ofthe source electrode and the drain electrode, and the conductive layer120 b that functions as the other can each be formed using a metalelement selected from chromium, copper, aluminum, gold, silver, zinc,molybdenum, tantalum, titanium, tungsten, manganese, nickel, iron, andcobalt; an alloy containing the metal element as its component; an alloyincluding a combination of the metal elements; or the like.

An oxide conductor or a metal oxide film such as an In—Sn oxide, an In—Woxide, an In—W—Zn oxide, an In—Ti oxide, an In—Ti—Sn oxide, an In—Znoxide, an In—Sn—Si oxide, or an In—Ga—Zn oxide can also be applied toeach of the conductive layer 112, the conductive layer 106, theconductive layer 120 a, and the conductive layer 120 b.

Here, an oxide conductor (OC) is described. For example, when oxygenvacancies are formed in a metal oxide having semiconductorcharacteristics and hydrogen is added to the oxygen vacancies, a donorlevel is formed in the vicinity of the conduction band. As a result, theconductivity of the metal oxide is increased, so that the metal oxidebecomes a conductor. The metal oxide having become a conductor can bereferred to as an oxide conductor.

In addition, the conductive layer 112 or the like may have astacked-layer structure of a conductive film containing the oxideconductor (the metal oxide) and a conductive film containing a metal oran alloy. The use of a conductive film containing a metal or an alloycan reduce wiring resistance. In that case, a conductive film containingan oxide conductor is preferably applied to a conductive film on a sidein contact with an insulating layer functioning as a gate insulatingfilm.

Furthermore, among the above metal elements, it is particularlypreferable that any one or more metal elements selected from titanium,tungsten, tantalum, and molybdenum be included in the conductive layer112, the conductive layer 106, the conductive layer 120 a, and theconductive layer 120 b. It is particularly preferable to use a tantalumnitride film. The tantalum nitride film has conductivity, has a highbarrier property against copper, oxygen, or hydrogen, and releaseslittle hydrogen from itself; thus, the tantalum nitride film can besuitably used as a conductive film that is in contact with thesemiconductor layer 108 or a conductive film that is in the vicinity ofthe semiconductor layer 108.

<Insulating Layer 110>

The insulating layer 110 functioning as a gate insulating film of thetransistor 100 or the like can be formed by a PECVD method, a sputteringmethod, or the like. For the insulating layer 110, an insulating layerincluding one or more of a silicon oxide film, a silicon oxynitridefilm, a silicon nitride oxide film, a silicon nitride film, an aluminumoxide film, a hafnium oxide film, an yttrium oxide film, a zirconiumoxide film, a gallium oxide film, a tantalum oxide film, a magnesiumoxide film, a lanthanum oxide film, a cerium oxide film, and a neodymiumoxide film can be used. Note that the insulating layer 110 may have astacked-layer structure of two layers or a stacked-layer structure ofthree or more layers.

In addition, the insulating layer 110 that is in contact with thesemiconductor layer 108 is preferably an oxide insulating film andfurther preferably includes a region containing oxygen in excess of thatin the stoichiometric composition. In other words, the insulating layer110 is an insulating film capable of releasing oxygen. It is alsopossible to supply oxygen into the insulating layer 110 by forming theinsulating layer 110 in an oxygen atmosphere, performing heat treatment,plasma treatment, or the like on the deposited insulating layer 110 inan oxygen atmosphere, or depositing an oxide film over the insulatinglayer 110 in an oxygen atmosphere, for example.

For the insulating layer 110, a material having a higher relativepermittivity than silicon oxide or silicon oxynitride, such as hafniumoxide, can also be used. In that case, the insulating layer 110 can bethick and leakage current due to tunnel current can be inhibited. Inparticular, hafnium oxide having crystallinity is preferable because ithas a higher relative permittivity than amorphous hafnium oxide.

<Semiconductor Layer>

In the case where the semiconductor layer 108 is an In-M-Zn oxide, asputtering target used for depositing the In-M-Zn oxide preferably hasthe atomic ratio of In to M is higher than or equal to 1. Examples ofthe atomic ratio of metal elements in such a sputtering target includesIn:M:Zn=1:1:1, In:MZn=1:1:1.2, In:M:Zn=2:1:3, In:MZn=3:1:2,In:M:Zn=4:2:3, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6, In:M:Zn=5:1:7,In:M:Zn=5:1:8, In:M:Zn=6:1:6, and In:M:Zn=5:2:5.

In addition, a target containing a polycrystalline oxide is preferablyused as the sputtering target, which facilitates formation of thesemiconductor layer 108 having crystallinity. Note that the atomic ratioin the semiconductor layer 108 to be deposited varies within the rangeof ±40% from the atomic ratio of the metal elements contained in thesputtering target. For example, in the case where the composition of asputtering target used for the semiconductor layer 108 isIn:Ga:Zn=4:2:4.1 [atomic ratio], the composition of the semiconductorlayer 108 to be deposited is in the neighborhood of In:Ga:Zn=4:2:3[atomic ratio] in some cases.

Note that when the atomic ratio is described as In:Ga:Zn=4:2:3 or asbeing in the neighborhood thereof, the case is included where Ga isgreater than or equal to 1 and less than or equal to 3 and Zn is greaterthan or equal to 2 and less than or equal to 4 with In being 4. Inaddition, when the atomic ratio is described as In:Ga:Zn=5:1:6 or asbeing in the neighborhood thereof, the case is included where Ga isgreater than 0.1 and less than or equal to 2 and Zn is greater than orequal to 5 and less than or equal to 7 with In being 5. Furthermore,when the atomic ratio is described as In:Ga:Zn=1:1:1 or as being in theneighborhood thereof, the case is included where Ga is greater than 0.1and less than or equal to 2 and Zn is greater than 0.1 and less than orequal to 2 with In being 1.

In addition, the energy gap of the semiconductor layer 108 is greaterthan or equal to 2 eV, preferably greater than or equal to 2.5 eV. Withthe use of such a metal oxide having a wider energy gap than silicon,the off-state current of the transistor can be reduced.

Furthermore, the semiconductor layer 108 preferably has anon-single-crystal structure. Examples of the non-single-crystalstructure include a CAAC structure to be described later, apolycrystalline structure, a microcrystalline structure, and anamorphous structure. Among the non-single-crystal structures, theamorphous structure has the highest density of defect states, whereasthe CAAC structure has the lowest density of defect states.

A CAAC (c-axis aligned crystal) will be described below. A CAAC refersto an example of a crystal structure.

The CAAC structure is a crystal structure of a thin film or the likethat has a plurality of nanocrystals (crystal regions having a maximumdiameter of less than 10 nm), characterized in that the nanocrystalseach have c-axis alignment in a particular direction, the nanocrystalseach have neither a-axis alignment nor b-axis alignment, and thenanocrystals have continuous crystal connection in the a-axis and b-axisdirections without forming a grain boundary. In particular, a thin filmhaving the CAAC structure is characterized in that the c-axes ofnanocrystals are likely to be aligned in a film thickness direction, anormal direction of a surface where the thin film is formed, or a normaldirection of a surface of the thin film.

A CAAC-OS (Oxide Semiconductor) is an oxide semiconductor with highcrystallinity. Meanwhile, in the CAAC-OS, it can be said that areduction in electron mobility due to the crystal grain boundary is lesslikely to occur because a clear crystal grain boundary cannot beobserved. Furthermore, the mixing of impurities, formation of defects,or the like might decrease the crystallinity of the oxide semiconductor;thus, it can also be said that the CAAC-OS is an oxide semiconductorhaving small amounts of impurities and defects (oxygen vacancies or thelike). Thus, an oxide semiconductor including a CAAC-OS is physicallystable. Therefore, the oxide semiconductor including a CAAC-OS isresistant to heat and has high reliability.

Here, in crystallography, in a unit cell formed with three axes (crystalaxes) of the a-axis, the b-axis, and the c-axis, a specific axis isgenerally taken as the c-axis. In particular, in the case of a crystalhaving a layered structure, two axes parallel to the plane direction ofa layer are regarded as the a-axis and the b-axis and an axisintersecting with the layer is regarded as the c-axis in general.Typical examples of such a crystal having a layered structure includegraphite, which is classified as a hexagonal system. In a unit cell ofgraphite, the a-axis and the b-axis are parallel to a cleavage plane andthe c-axis is orthogonal to the cleavage plane. For example, an InGaZnO₄crystal having a YbFe₂O₄ type crystal structure, which is a layeredstructure, can be classified as a hexagonal system, and in a unit cellthereof, the a-axis and the b-axis are parallel to the plane directionof a layer and the c-axis is orthogonal to the layer (i.e., the a-axisand the b-axis).

In an image observed with a TEM, crystal parts cannot be found clearlyin an oxide semiconductor film having a microcrystalline structure (amicrocrystalline oxide semiconductor film) in some cases. In most cases,the size of a crystal part included in the microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. In particular, an oxide semiconductor film including ananocrystal (nc) that is a microcrystal with a size greater than orequal to 1 nm and less than or equal to 10 nm, or greater than or equalto 1 nm and less than or equal to 3 nm is referred to as an nc-OS(nanocrystalline Oxide Semiconductor) film. In an image observed with aTEM, for example, a grain boundary cannot be found clearly in the nc-OSfilm in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. Furthermore,there is no regularity of crystal orientation between different crystalparts in the nc-OS film. Thus, the orientation in the whole film is notobserved. Accordingly, in some cases, the nc-OS film cannot bedistinguished from an amorphous oxide semiconductor film depending on ananalysis method. For example, when the nc-OS film is subjected tostructural analysis by an out-of-plane method with an XRD apparatususing an X-ray having a diameter larger than the size of a crystal part,a peak that shows a crystal plane does not appear. Furthermore, adiffraction pattern like a halo pattern is observed when the nc-OS filmis subjected to electron diffraction (also referred to as selected-areaelectron diffraction) using an electron beam with a probe diameter(e.g., 50 nm or larger) that is larger than a crystal part. Meanwhile,in some cases, a circular (ring-like) region with high luminance isobserved when electron diffraction (also referred to as nanobeamelectron diffraction) using an electron beam with a probe diameter(e.g., 1 nm or larger and 30 nm or smaller) close to or smaller than thesize of a crystal part is performed on the nc-OS film, and a pluralityof spots are observed in the region.

The nc-OS film has a lower density of defect states than an amorphousoxide semiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film. Thus, thenc-OS film has a higher density of defect states than the CAAC-OS film.Accordingly, the nc-OS film has a higher carrier density and higherelectron mobility than the CAAC-OS film in some cases. Therefore, atransistor including the nc-OS film may have high field-effect mobility.

The nc-OS film can be formed at a lower oxygen flow rate ratio in filmformation than the CAAC-OS film. The nc-OS film can also be formed at alower substrate temperature in film formation than the CAAC-OS film. Forexample, the nc-OS film can be formed at a relatively low substratetemperature (e.g., a temperature of 130° C. or lower) or without heatingof the substrate and thus is suitable for the case of using a largeglass substrate, a resin substrate, or the like, and productivity can beincreased.

An example of a crystal structure of a metal oxide is described. Notethat a metal oxide formed by a sputtering method using an In—Ga—Zn oxidetarget (In:Ga:Zn=4:2:4.1 [atomic ratio]) is described below as anexample. A metal oxide that is formed by a sputtering method using theabove target at a substrate temperature higher than or equal to 100° C.and lower than or equal to 130° C. is likely to have either the nc (nanocrystal) structure or the CAAC structure, or a structure in which bothstructures are mixed. By contrast, a metal oxide formed by a sputteringmethod at a substrate temperature set at room temperature (R.T.) islikely to have the nc structure. Note that room temperature (R. T.) herealso includes a temperature in the case where a substrate is not heatedintentionally.

[Composition of Metal Oxide]

The composition of a CAC (Cloud-Aligned Composite)-OS that can be usedin a transistor disclosed in one embodiment of the present inventionwill be described below.

Note that in this specification and the like, “CAAC (c-axis alignedcrystal)” or “CAC (Cloud-Aligned Composite)” might be stated. Note thatCAAC refers to an example of a crystal structure, and CAC refers to anexample of a function or a material composition.

A CAC-OS or a CAC-metal oxide has a conducting function in part of thematerial and has an insulating function in another part of the material;as a whole, the CAC-OS or the CAC-metal oxide has a function of asemiconductor. Note that in the case where the CAC-OS or the CAC-metaloxide is used in an active layer of a transistor, the conductingfunction is a function that allows electrons (or holes) serving ascarriers to flow, and the insulating function is a function that doesnot allow electrons serving as carriers to flow. By the complementaryaction of the conducting function and the insulating function, aswitching function (On/Off function) can be given to the CAC-OS or theCAC-metal oxide. In the CAC-OS or the CAC-metal oxide, separation of thefunctions can maximize each function.

The CAC-OS or the CAC-metal oxide includes conductive regions andinsulating regions. The conductive regions have the above-describedconducting function, and the insulating regions have the above-describedinsulating function. Furthermore, in some cases, the conductive regionsand the insulating regions in the material are separated at thenanoparticle level. Furthermore, in some cases, the conductive regionsand the insulating regions are unevenly distributed in the material. Theconductive regions are observed to be coupled in a cloud-like mannerwith their boundaries blurred, in some cases.

In the CAC-OS or the CAC-metal oxide, the conductive regions and theinsulating regions each have a size greater than or equal to 0.5 nm andless than or equal to 10 nm, preferably greater than or equal to 0.5 nmand less than or equal to 3 nm, and are dispersed in the material, insome cases.

The CAC-OS or the CAC-metal oxide includes components having differentband gaps. For example, the CAC-OS or the CAC-metal oxide includes acomponent having a wide gap due to the insulating region and a componenthaving a narrow gap due to the conductive region. In the case of such astructure, when carriers flow, carriers mainly flow in the componenthaving a narrow gap. Furthermore, the component having a narrow gapcomplements the component having a wide gap, and carriers also flow inthe component having a wide gap in conjunction with the component havinga narrow gap. Therefore, in the case where the above-described CAC-OS orCAC-metal oxide is used in a channel formation region of a transistor,high current driving capability in an on state of the transistor, thatis, a high on-state current and high field-effect mobility, can beobtained.

In other words, the CAC-OS or the CAC-metal oxide can also be referredto as a matrix composite or a metal matrix composite.

The above is the description of the components.

At least part of this embodiment can be implemented in combination withthe other embodiments described in this specification as appropriate.

Embodiment 2

In this embodiment, an example of a display device that includes any ofthe transistors described in the above embodiment will be described.

Structure Example

FIG. 9(A) is a top view of a display device 700. The display device 700includes a first substrate 701 and a second substrate 705 that areattached to each other with a sealant 712. In a region sealed with thefirst substrate 701, the second substrate 705, and the sealant 712, apixel portion 702, a source driver circuit portion 704, and a gatedriver circuit portion 706 are provided over the first substrate 701. Inthe pixel portion 702, a plurality of display elements are provided.

An FPC terminal portion 708 to which an FPC 716 (FPC: Flexible printedcircuit) is connected is provided in a portion of the first substrate701 that does not overlap with the second substrate 705. The pixelportion 702, the source driver circuit portion 704, and the gate drivercircuit portion 706 are each supplied with a variety of signals and thelike from the FPC 716 through the FPC terminal portion 708 and a signalline 710.

A plurality of gate driver circuit portions 706 may be provided. Thegate driver circuit portion 706 and the source driver circuit portion704 may be formed separately on semiconductor substrates or the like toobtain packaged IC chips. The IC chips can each be mounted on the firstsubstrate 701 or the FPC 716.

Any of the transistors that are the semiconductor devices of embodimentsof the present invention can be used as transistors included in thepixel portion 702, the source driver circuit portion 704, and the gatedriver circuit portion 706.

Examples of the display element provided in the pixel portion 702include a liquid crystal element and a light-emitting element. As theliquid crystal element, a transmissive liquid crystal element, areflective liquid crystal element, a transflective liquid crystalelement, or the like can be used. As the light-emitting element, aself-luminous light-emitting element such as an LED (Light EmittingDiode), an OLED (Organic LED), a QLED (Quantum-dot LED), or asemiconductor laser can be used. It is also possible to use a MEMS(Micro Electro Mechanical Systems) shutter element, an opticalinterference type MEMS element, or a display element using amicrocapsule method, an electrophoretic method, an electrowettingmethod, an Electronic Liquid Powder (registered trademark) method, orthe like, for instance.

A display device 700A illustrated in FIG. 9(B) is an example of adisplay device which includes a flexible resin layer 743 instead of thefirst substrate 701 and can be used as a flexible display.

In the display device 700A, the pixel portion 702 has not a rectangularshape but a shape with rounded corners. The display device 700A includesa notch portion in which part of the pixel portion 702 and part of theresin layer 743 are cut as illustrated in a region P1 in FIG. 9(B). Apair of gate driver circuit portions 706 is provided on the oppositesides with the pixel portion 702 therebetween. The gate driver circuitportions 706 are provided along a curved outline at the corners of thepixel portion 702.

The resin layer 743 has a shape with a sticking-out portion where theFPC terminal portion 708 is provided. Furthermore, part of the resinlayer 743 that includes the FPC terminal portion 708 can be bentbackward in a region P2 in FIG. 9(B). When part of the resin layer 743is bent backward, the display device 700A can be mounted on anelectronic device while the FPC 716 overlaps with the back side of thepixel portion 702; thus, the electronic device can be downsized.

An IC 717 is mounted on the FPC 716 connected to the display device700A. The IC 717 functions as a source driver circuit, for example. Inthis case, the source driver circuit portion 704 in the display device700A can include at least one of a protection circuit, a buffer circuit,a demultiplexer circuit, and the like.

A display device 700B illustrated in FIG. 9(C) is a display device thatcan be suitably used for an electronic device with a large screen. Forexample, the display device 700B can be suitably used for a televisiondevice, a monitor device, a personal computer (including a notebook typeand a desktop type), a tablet terminal, digital signage, or the like.

The display device 700B includes a plurality of source driver ICs 721and a pair of gate driver circuit portions 722.

The plurality of source driver ICs 721 are attached to respective FPCs723. In each of the plurality of FPCs 723, one of terminals is connectedto the first substrate 701, and the other terminal is connected to aprinted circuit board 724. By bending the FPCs 723, the printed circuitboard 724 can be placed on the back side of the pixel portion 702 sothat the display device 700B can be mounted on an electronic device;thus, the electronic device can be downsized.

By contrast, the gate driver circuit portions 722 are provided over thefirst substrate 701. Thus, an electronic device with a narrow bezel canbe provided.

With such a structure, a large-size and high-resolution display devicecan be provided. For example, a display device with a diagonal screensize of 30 inches or more, 40 inches or more, 50 inches or more, or 60inches or more can be obtained. Furthermore, a display device withextremely high resolution such as 4K2K or 8K4K can be provided.

[Cross-Sectional Structure Example]

Structures using a liquid crystal element as a display element andstructures using an EL element will be described below with reference toFIG. 10 to FIG. 13 . Note that FIG. 10 to FIG. 12 are cross-sectionalviews taken along dashed-dotted line Q-R in FIG. 9(A). FIG. 13 is across-sectional view taken along dashed-dotted line S-T in the displaydevice 700A in FIG. 9(B). FIG. 10 and FIG. 11 are each a structure usinga liquid crystal element as a display element, and FIG. 12 and FIG. 13are each a structure using an EL element.

<Description of Common Portions in Display Devices>

Display devices in FIG. 10 to FIG. 13 each include a lead wiring portion711, the pixel portion 702, the source driver circuit portion 704, andthe FPC terminal portion 708. The lead wiring portion 711 includes thesignal line 710. The pixel portion 702 includes a transistor 750 and acapacitor 790. The source driver circuit portion 704 includes atransistor 752. FIG. 11 illustrates a case where the capacitor 790 isnot provided.

As the transistor 750 and the transistor 752, any of the transistorsdescribed in Embodiment 1 can be used.

The transistor used in this embodiment includes a highly purified oxidesemiconductor film in which formation of oxygen vacancies is suppressed.The transistor can have low off-state current. Accordingly, anelectrical signal such as an image signal can be held for a longerperiod, and the interval between writes of an image signal or the likecan be set longer. Thus, frequency of refresh operation can be reduced,which leads to lower power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high-speed operation.For example, with such a transistor capable of high-speed operation usedfor a display device, a switching transistor in a pixel portion and adriver transistor used in a driver circuit portion can be formed overone substrate. That is, a structure in which a driver circuit formedusing a silicon wafer or the like is not used is possible, in which casethe number of components of the display device can be reduced. Moreover,the use of the transistor capable of high-speed operation also in thepixel portion can provide a high-quality image.

The capacitor 790 illustrated in FIG. 10 , FIG. 12 , and FIG. 13includes a lower electrode formed by processing the same film as a firstgate electrode included in the transistor 750 and an upper electrodeformed by processing the same metal oxide film as the semiconductorlayer. The upper electrode has reduced resistance like a source regionand a drain region of the transistor 750. Part of an insulating filmfunctioning as a first gate insulating layer of the transistor 750 isprovided between the lower electrode and the upper electrode. That is,the capacitor 790 has a stacked-layer structure in which the insulatingfilms functioning as dielectric films are positioned between a pair ofelectrodes. A wiring obtained by processing the same film as a sourceelectrode and a drain electrode of the transistor is connected to theupper electrode.

A planarization insulating film 770 is provided over the transistor 750,the transistor 752, and the capacitor 790.

The transistor 750 in the pixel portion 702 and the transistor 752 inthe source driver circuit portion 704 may have different structures. Forexample, a top-gate transistor may be used as one of the transistors 750and 752, and a bottom-gate transistor may be used as the other. Notethat the same can be said for the gate driver circuit portion 706, asthe source driver circuit portion 704.

The signal line 710 is formed using the same conductive film as thesource electrodes, the drain electrodes, and the like of the transistors750 and 752. In this case, a low-resistance material such as a materialcontaining a copper element is preferably used because signal delay orthe like due to the wiring resistance can be reduced and display on alarge screen is possible.

The FPC terminal portion 708 includes a wiring 760 part of whichfunctions as a connection electrode, an anisotropic conductive film 780,and the FPC 716. The wiring 760 is electrically connected to a terminalincluded in the FPC 716 through the anisotropic conductive film 780. Thewiring 760 is formed using the same conductive film as the sourceelectrodes, the drain electrodes, and the like of the transistors 750and 752.

As the first substrate 701 and the second substrate 705, a glasssubstrate or a flexible substrate such as a plastic substrate can beused, for example. In the case where a flexible substrate is used as thefirst substrate 701, an insulating layer having a barrier propertyagainst water or hydrogen is preferably provided between the firstsubstrate 701 and the transistor 750, for example.

Alight-blocking film 738, a coloring film 736, and an insulating film734 in contact with these films are provided on the second substrate 705side.

<Structure Example of Display Device Using Liquid Crystal Element>

The display device 700 illustrated in FIG. 10 includes a liquid crystalelement 775 and a spacer 778. The liquid crystal element 775 includes aconductive layer 772, a conductive layer 774, and a liquid crystal layer776 therebetween. The conductive layer 774 is provided on the secondsubstrate 705 side and has a function of a common electrode. Theconductive layer 772 is electrically connected to the source electrodeor the drain electrode of the transistor 750. The conductive layer 772is formed over the planarization insulating film 770 and functions as apixel electrode.

A material that transmits visible light or a material that reflectsvisible light can be used for the conductive layer 772. As alight-transmitting material, for example, an oxide material includingindium, zinc, tin, or the like is preferably used. As a reflectivematerial, for example, a material including aluminum, silver, or thelike is preferably used.

When a reflective material is used for the conductive layer 772, thedisplay device 700 is a reflective liquid crystal display device. When alight-transmitting material is used for the conductive layer 772, atransmissive liquid crystal display device is obtained. For a reflectiveliquid crystal display device, a polarizing plate is provided on theviewer side. By contrast, for a transmissive liquid crystal displaydevice, a pair of polarizing plates is provided so that the liquidcrystal element is placed therebetween.

The display device 700 in FIG. 11 is an example of employing the liquidcrystal element 775 of a horizontal electric field mode (e.g., an FFSmode). The conductive layer 774 functioning as a common electrode isprovided over the conductive layer 772 with an insulating layer 773therebetween. An electric field generated between the conductive layer772 and the conductive layer 774 can control the alignment state in theliquid crystal layer 776.

In FIG. 11 , a storage capacitor can be formed with a stacked-layerstructure including the conductive layer 774, the insulating layer 773,and the conductive layer 772. Thus, another capacitor need not beprovided, and thus the aperture ratio can be increased.

Although not illustrated in FIG. 10 and FIG. 11 , a structure in whichan alignment film in contact with the liquid crystal layer 776 isprovided may be employed. Furthermore, an optical member (an opticalsubstrate) such as a polarizing member, a retardation member, or ananti-reflection member, and a light source such as a backlight or asidelight can be provided as appropriate.

For the liquid crystal layer 776, a thermotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, a polymerdispersed liquid crystal (PDLC), a polymer network liquid crystal(PNLC), a ferroelectric liquid crystal, an anti-ferroelectric liquidcrystal, or the like can be used. In the case where a horizontalelectric field mode is employed, a liquid crystal exhibiting a bluephase for which an alignment film is not used may be used.

The following can be used as a mode of the liquid crystal element: a TN(Twisted Nematic) mode, a VA (Vertical Alignment) mode, an IPS(In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an ASM(Axially Symmetric aligned Micro-cell) mode, an OCB (OpticallyCompensated Birefringence) mode, an ECB (Electrically ControlledBirefringence) mode, a guest-host mode, or the like.

In addition, a scattering liquid crystal element employing a polymerdispersed liquid crystal, a polymer network liquid crystal, or the likecan be used for the liquid crystal layer 776. At this time, monochromeimage display may be performed without the coloring film 736, or colordisplay may be performed using the coloring film 736.

As a method for driving the liquid crystal element, a time-divisiondisplay method (also referred to as a field sequential driving method)in which color display is performed on the basis of a successiveadditive color mixing method may be employed. In that case, a structurein which the coloring film 736 is not provided may be employed. In thecase where the time-division display method is employed, advantages suchas the aperture ratio of each pixel or the resolution being increasedcan be obtained because subpixels that emit light of, for example, R(red), G (green), and B (blue), need not be provided.

<Display Device Using Light-Emitting Element>

The display device 700 illustrated in FIG. 12 includes a light-emittingelement 782. The light-emitting element 782 includes the conductivelayer 772, an EL layer 786, and a conductive film 788. The EL layer 786contains an organic compound or an inorganic compound such as quantumdots.

Examples of materials that can be used for an organic compound include afluorescent material and a phosphorescent material. Examples ofmaterials that can be used for quantum dots include a colloidal quantumdot material, an alloyed quantum dot material, a core-shell quantum dotmaterial, and a core quantum dot material.

In the display device 700 illustrated in FIG. 12 , an insulating film730 covering part of the conductive layer 772 is provided over theplanarization insulating film 770. Here, the light-emitting element 782is a top-emission light-emitting element, which includes the conductivefilm 788 with a light-transmitting property. Note that thelight-emitting element 782 may have a bottom-emission structure in whichlight is emitted to the conductive layer 772 side, or a dual-emissionstructure in which light is emitted to both the conductive layer 772side and the conductive film 788 side.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided in the leadwiring portion 711, the source driver circuit portion 704, and aposition overlapping with the insulating film 730. The coloring film 736and the light-blocking film 738 are covered with the insulating film734. A space between the light-emitting element 782 and the insulatingfilm 734 is filled with a sealing film 732. Note that a structure inwhich the coloring film 736 is not provided may be employed when the ELlayer 786 is formed into an island shape for each pixel or into a stripeshape for each pixel column, i.e., the EL layer 786 is formed byseparate coloring.

FIG. 13 illustrates a structure of a display device suitably applicableto a flexible display. FIG. 13 is a cross-sectional view taken along thedashed-dotted line S-T in the display device 700A in FIG. 9(B).

The display device 700A in FIG. 13 has a structure in which a supportsubstrate 745, a bonding layer 742, the resin layer 743, and aninsulating layer 744 are stacked instead of the first substrate 701 inFIG. 12 . The transistor 750, the capacitor 790, and the like areprovided over the insulating layer 744 over the resin layer 743.

The support substrate 745 includes an organic resin, glass, or the likeand is thin enough to have flexibility. The resin layer 743 is a layercontaining an organic resin such as polyimide or acrylic. The insulatinglayer 744 includes an inorganic insulating film of silicon oxide,silicon oxynitride, silicon nitride, or the like. The resin layer 743and the support substrate 745 are attached to each other with thebonding layer 742. The resin layer 743 is preferably thinner than thesupport substrate 745.

The display device 700A in FIG. 13 includes a protective layer 740instead of the substrate 705 in FIG. 12 . The protective layer 740 isattached to the sealing film 732. A glass substrate, a resin film, orthe like can be used as the protective layer 740. Alternatively, as theprotective layer 740, an optical member such as a polarizing plate or ascattering plate, an input device such as a touch sensor panel, or astructure in which two or more of the above are stacked may be employed.

The EL layer 786 included in the light-emitting element 782 is providedover the insulating film 730 and the conductive layer 772 in an islandshape. The EL layers 786 are formed separately so that respectivesubpixels emit light of different colors, whereby color display can beperformed without use of the coloring film 736. A protective layer 741is provided to cover the light-emitting element 782. The protectivelayer 741 has a function of preventing diffusion of impurities such aswater into the light-emitting element 782. The protective layer 741 ispreferably formed using an inorganic insulating film. The protectivelayer 741 further preferably has a stacked-layer structure including oneor more inorganic insulating films and one or more organic insulatingfilms.

FIG. 13 illustrates the region P2 that can be bent. The region P2includes a portion where the support substrate 745, the bonding layer742, and the inorganic insulating film such as the insulating layer 744are not provided. In the region P2, a resin layer 746 is provided tocover the wiring 760. When a structure is employed in which an inorganicinsulating film is not provided if possible in the region P2 that can bebent and only a conductive layer containing a metal or an alloy and alayer containing an organic material are stacked, generation of crackscaused at bending can be prevented. When the support substrate 745 isnot provided in the region P2, part of the display device 700A can bebent with an extremely small radius of curvature.

<Structure Example of Display Device Provided with Input Device>

An input device may be provided in the display device illustrated inFIG. 10 to FIG. 13 . Examples of the input device include a touchsensor.

A variety of types such as a capacitive type, a resistive type, asurface acoustic wave type, an infrared type, an optical type, and apressure-sensitive type can be used as the sensor type, for example.Alternatively, two or more of these types may be combined and used.

Examples of the touch panel structure include what is called an in-celltouch panel in which an input device is provided between a pair ofsubstrates, what is called an on-cell touch panel in which an inputdevice is formed over the display device, or what is called an out-celltouch panel in which an input device is attached to the display device.

At least part of the structure examples, the drawings correspondingthereto, and the like exemplified in this embodiment can be implementedin combination with the other structure examples, the other drawings,and the like as appropriate.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 3

In this embodiment, a display device that includes the semiconductordevice of one embodiment of the present invention will be described withreference to FIG. 14 .

A display device illustrated in FIG. 14(A) includes a pixel portion 502,a driver circuit portion 504, protection circuits 506, and a terminalportion 507. Note that a structure in which the protection circuits 506are not provided may be employed.

The transistor of one embodiment of the present invention can be used astransistors included in the pixel portion 502 and the driver circuitportion 504. The transistor of one embodiment of the present inventionmay also be used in the protection circuits 506.

The pixel portion 502 includes a plurality of pixel circuits 501 thatdrive a plurality of display elements arranged in X rows and Y columns(X and Y each independently represent a natural number of 2 or more).

The driver circuit portion 504 includes driver circuits such as a gatedriver 504 a that outputs a scanning signal to scan lines GL_1 to GL_Xand a source driver 504 b that supplies a data signal to data lines DL_1to DL Y. The gate driver 504 a includes at least a shift register. Thesource driver 504 b is formed using a plurality of analog switches, forexample. Alternatively, the source driver 504 b may be formed using ashift register or the like.

The terminal portion 507 refers to a portion provided with terminals forinputting power, control signals, image signals, and the like to thedisplay device from external circuits.

The protection circuit 506 is a circuit that, when a potential out of acertain range is applied to a wiring to which the protection circuit 506is connected, establishes continuity between the wiring and anotherwiring. The protection circuit 506 illustrated in FIG. 14(A) isconnected to a variety of wirings such as the scan lines GL that arewirings between the gate driver 504 a and the pixel circuits 501 and thedata lines DL that are wirings between the source driver 504 b and thepixel circuits 501, for example.

The gate driver 504 a and the source driver 504 b may be provided over asubstrate over which the pixel portion 502 is provided, or a substratewhere a gate driver circuit or a source driver circuit is separatelyformed (e.g., a driver circuit board formed using a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted on the substrate by COG or TAB (Tape Automated Bonding).

The plurality of pixel circuits 501 illustrated in FIG. 14(A) can have aconfiguration illustrated in FIG. 14(B) or FIG. 14(C), for example.

The pixel circuit 501 illustrated in FIG. 14(B) includes a liquidcrystal element 570, a transistor 550, and a capacitor 560. The dataline DL n, the scan line GL_m, a potential supply line VL, and the likeare connected to the pixel circuit 501.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set appropriately in accordance with the specificationsof the pixel circuit 501. The alignment state of the liquid crystalelement 570 is set depending on written data. Note that a commonpotential may be supplied to one of the pair of electrodes of the liquidcrystal element 570 included in each of the plurality of pixel circuits501. Alternatively, a potential supplied to one of the pair ofelectrodes of the liquid crystal element 570 of the pixel circuit 501may differ between rows.

The pixel circuit 501 illustrated in FIG. 14(C) includes transistors 552and 554, a capacitor 562, and a light-emitting element 572. The dataline DL n, the scan line GL_m, a potential supply line VL_a, a powersupply line VL_b, and the like are connected to the pixel circuit 501.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other. Current flowingthrough the light-emitting element 572 is controlled in accordance witha potential applied to a gate of the transistor 554, whereby theluminance of light emitted from the light-emitting element 572 iscontrolled.

At least part of the structure examples, the drawings correspondingthereto, and the like exemplified in this embodiment can be implementedin combination with the other structure examples, the other drawings,and the like as appropriate.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

A pixel circuit including a memory for correcting gray levels displayedby pixels and a display device including the pixel circuit will bedescribed below. The transistor described in Embodiment 1 can be used asa transistor used in the pixel circuit described below.

[Circuit Configuration]

FIG. 15(A) is a circuit diagram of a pixel circuit 400. The pixelcircuit 400 includes a transistor M1, a transistor M2, a capacitor C1,and a circuit 401. A wiring S1, a wiring S2, a wiring G1, and a wiringG2 are connected to the pixel circuit 400.

In the transistor M1, a gate is connected to the wiring G1, one of asource and a drain is connected to the wiring S1, and the other isconnected to one electrode of the capacitor C1. In the transistor M2, agate is connected to the wiring G2, one of a source and a drain isconnected to the wiring S2, and the other is connected to the otherelectrode of the capacitor C1 and the circuit 401.

The circuit 401 is a circuit including at least one display element. Anyof a variety of elements can be used as the display element, andtypically, a light-emitting element such as an organic EL element or anLED element, a liquid crystal element, a MEMS (Micro Electro MechanicalSystems) element, or the like can be used.

Anode connecting the transistor M1 and the capacitor C1 is denoted as anode N1, and a node connecting the transistor M2 and the circuit 401 isdenoted as a node N2.

In the pixel circuit 400, the potential of the node N1 can be retainedwhen the transistor M1 is turned off. The potential of the node N2 canbe retained when the transistor M2 is turned off. When a predeterminedpotential is written to the node N1 through the transistor M1 with thetransistor M2 being in an off state, the potential of the node N2 can bechanged in accordance with displacement of the potential of the node N1owing to capacitive coupling through the capacitor C1.

Here, the transistor using an oxide semiconductor, which is described inEmbodiment 1, can be used as one or both of the transistor M1 and thetransistor M2. Accordingly, owing to an extremely low off-state current,the potentials of the node N1 and the node N2 can be retained for a longtime. Note that in the case where the period in which the potential ofeach node is retained is short (specifically, the case where the framefrequency is higher than or equal to 30 Hz, for example), a transistorusing a semiconductor such as silicon may be used.

[Driving Method Example]

Next, an example of a method for operating the pixel circuit 400 isdescribed with reference to FIG. 15(B). FIG. 15(B) is a timing chart ofthe operation of the pixel circuit 400. Note that for simplification ofdescription, the influence of various kinds of resistance such as wiringresistance, parasitic capacitance of a transistor, a wiring, or thelike, the threshold voltage of the transistor, and the like is not takeninto account here.

In the operation shown in FIG. 15(B), one frame period is divided into aperiod T1 and a period T2. The period T1 is a period in which apotential is written to the node N2, and the period T2 is a period inwhich a potential is written to the node N1.

<Period T1>

In the period T1, a potential for turning on the transistor is suppliedto both the wiring G1 and the wiring G2. In addition, a potentialV_(ref) that is a fixed potential is supplied to the wiring S1, and afirst data potential V_(w) is supplied to the wiring S2.

The potential V_(ref) is supplied from the wiring S1 to the node N1through the transistor M1. The first data potential V_(w) is supplied tothe node N2 through the transistor M2. Accordingly, a potentialdifference V_(w)−V_(ref) is retained in the capacitor C1.

<Period T2>

Next, in the period T2, a potential for turning on the transistor M1 issupplied to the wiring G1, and a potential for turning off thetransistor M2 is supplied to the wiring G2. A second data potentialV_(data) is supplied to the wiring S1. The wiring S2 may be suppliedwith a predetermined constant potential or brought into a floatingstate.

The second data potential V_(data) is supplied to the node N1 throughthe transistor M1. At this time, capacitive coupling due to thecapacitor C1 changes the potential of the node N2 in accordance with thesecond data potential V_(data) by a potential dV. That is, a potentialthat is the sum of the first data potential V_(w) and the potential dVis input to the circuit 401. Note that although dV is shown as apositive value in FIG. 15(B), the potential dV may be a negative value.That is, the second data potential V_(data) may be lower than thepotential V_(ref).

Here, the potential dV is roughly determined by the capacitance of thecapacitor C1 and the capacitance of the circuit 401. When thecapacitance of the capacitor C1 is sufficiently larger than thecapacitance of the circuit 401, the potential dV is a potential close tothe second data potential V_(data).

In the above manner, the pixel circuit 400 can generate a potential tobe supplied to the circuit 401 including the display element, bycombining two kinds of data signals; hence, a gray level can becorrected in the pixel circuit 400.

The pixel circuit 400 can also generate a potential exceeding themaximum potential that can be supplied to the wiring S1 and the wiringS2. For example, in the case where a light-emitting element is used,high-dynamic range (HDR) display or the like can be performed. In thecase where a liquid crystal element is used, overdriving or the like canbe achieved.

APPLICATION EXAMPLES

<Example Using Liquid Crystal Element>

A pixel circuit 400LC illustrated in FIG. 15(C) includes a circuit401LC. The circuit 401LC includes a liquid crystal element LC and acapacitor C2.

In the liquid crystal element LC, one electrode is connected to the nodeN2 and one electrode of the capacitor C2, and the other electrode isconnected to a wiring supplied with a potential V_(com2). The otherelectrode of the capacitor C2 is connected to a wiring supplied with apotential V_(com1).

The capacitor C2 functions as a storage capacitor. Note that thecapacitor C2 can be omitted when not needed.

In the pixel circuit 400LC, a high voltage can be supplied to the liquidcrystal element LC; thus, high-speed display can be performed byoverdriving or a liquid crystal material with a high driving voltage canbe employed, for example. Moreover, by supply of a correction signal tothe wiring S1 or the wiring S2, a gray level can be corrected inaccordance with the operating temperature, the deterioration state ofthe liquid crystal element LC, or the like.

<Example Using Light-Emitting Element>

A pixel circuit 400EL illustrated in FIG. 15(D) includes a circuit401EL. The circuit 401EL includes a light-emitting element EL, atransistor M3, and the capacitor C2.

In the transistor M3, a gate is connected to the node N2 and oneelectrode of the capacitor C2, one of a source and a drain is connectedto a wiring supplied with a potential V_(H), and the other is connectedto one electrode of the light-emitting element EL. The other electrodeof the capacitor C2 is connected to a wiring supplied with a potentialV_(com). The other electrode of the light-emitting element EL isconnected to a wiring supplied with a potential V_(L).

The transistor M3 has a function of controlling a current to be suppliedto the light-emitting element EL. The capacitor C2 functions as astorage capacitor. The capacitor C2 can be omitted when not needed.

Note that although the structure in which the anode side of thelight-emitting element EL is connected to the transistor M3 is describedhere, the transistor M3 may be connected to the cathode side. In thatcase, the values of the potential V_(H) and the potential V_(L) can beappropriately changed.

In the pixel circuit 400EL, a large amount of current can flow throughthe light-emitting element EL when a high potential is applied to thegate of the transistor M3, which enables HDR display, for example.Moreover, a variation in the electrical characteristics of thetransistor M3 and the light-emitting element EL can be corrected bysupply of a correction signal to the wiring S1 or the wiring S2.

Note that the configuration is not limited to the circuits illustratedin FIGS. 15(C) and 15(D), and a configuration to which a transistor, acapacitor, or the like is further added may be employed.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 5

In this embodiment, a display module that can be fabricated using oneembodiment of the present invention will be described.

In a display module 6000 illustrated in FIG. 16(A), a display device6006 to which an FPC 6005 is connected, a frame 6009, a printed circuitboard 6010, and a battery 6011 are provided between an upper cover 6001and a lower cover 6002.

A display device fabricated using one embodiment of the presentinvention can be used as the display device 6006, for example. With thedisplay device 6006, a display module with extremely low powerconsumption can be achieved.

The shape and size of the upper cover 6001 and the lower cover 6002 canbe changed as appropriate in accordance with the size of the displaydevice 6006.

The display device 6006 may have a function of a touch panel.

The frame 6009 may have a function of protecting the display device6006, a function of blocking electromagnetic waves generated by theoperation of the printed circuit board 6010, a function of a heatdissipation plate, or the like.

The printed circuit board 6010 includes a power supply circuit, a signalprocessing circuit for outputting a video signal and a clock signal, abattery control circuit, and the like.

FIG. 16(B) is a schematic cross-sectional view of the display module6000 having an optical touch sensor.

The display module 6000 includes a light-emitting portion 6015 and alight-receiving portion 6016 that are provided on the printed circuitboard 6010. Furthermore, a pair of light guide portions (a light guideportion 6017 a and a light guide portion 6017 b) are provided in regionssurrounded by the upper cover 6001 and the lower cover 6002.

The display device 6006 overlaps with the printed circuit board 6010 andthe battery 6011 with the frame 6009 therebetween. The display device6006 and the frame 6009 are fixed to the light guide portion 6017 a andthe light guide portion 6017 b.

Light 6018 emitted from the light-emitting portion 6015 travels over thedisplay device 6006 through the light guide portion 6017 a and reachesthe light-receiving portion 6016 through the light guide portion 6017 b.For example, blocking of the light 6018 by a sensing target such as afinger or a stylus enables detection of touch operation.

A plurality of light-emitting portions 6015 are provided along twoadjacent sides of the display device 6006, for example. A plurality oflight-receiving portions 6016 are provided at the positions on theopposite side of the light-emitting portions 6015. Accordingly,information about the position of touch operation can be obtained.

As the light-emitting portion 6015, a light source such as an LEDelement can be used, for example, and it is particularly preferable touse a light source emitting infrared rays. As the light-receivingportion 6016, a photoelectric element that receives light emitted fromthe light-emitting portion 6015 and converts it into an electric signalcan be used. A photodiode that can receive infrared rays can be suitablyused.

With the use of the light guide portion 6017 a and the light guideportion 6017 b which transmit the light 6018, the light-emitting portion6015 and the light-receiving portion 6016 can be placed under thedisplay device 6006, and a malfunction of the touch sensor due toexternal light reaching the light-receiving portion 6016 can besuppressed. Particularly when a resin that absorbs visible light andtransmits infrared rays is used, a malfunction of the touch sensor canbe suppressed more effectively.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 6

In this embodiment, examples of an electronic device for which thedisplay device of one embodiment of the present invention can be usedwill be described.

An electronic device 6500 illustrated in FIG. 17(A) is a portableinformation terminal that can be used as a smartphone.

The electronic device 6500 includes, in a housing 6501, a displayportion 6502, a power button 6503, buttons 6504, a speaker 6505, amicrophone 6506, a camera 6507, a light source 6508, and the like. Thedisplay portion 6502 has a touch panel function.

The display device of one embodiment of the present invention can beused in the display portion 6502.

FIG. 17(B) is a schematic cross-sectional view including an end portionof the housing 6501 on the microphone 6506 side.

A protective member 6510 having a light-transmitting property isprovided on the display surface side of the housing 6501, and a displaypanel 6511, an optical member 6512, a touch sensor panel 6513, a printedcircuit board 6517, a battery 6518, and the like are provided in a spacesurrounded by the housing 6501 and the protective member 6510.

The display panel 6511, the optical member 6512, and the touch sensorpanel 6513 are fixed to the protective member 6510 with a bonding layernot illustrated.

Part of the display panel 6511 is bent in a region outside the displayportion 6502. An FPC 6515 is connected to the bent part. An IC 6516 ismounted on the FPC 6515. The FPC 6515 is connected to a terminalprovided for the printed circuit board 6517.

A flexible display panel of one embodiment of the present invention canbe used as the display panel 6511. Thus, an extremely lightweightelectronic device can be achieved. Furthermore, since the display panel6511 is extremely thin, the battery 6518 with a high capacity can beprovided without an increase in the thickness of the electronic device.Moreover, part of the display panel 6511 is bent to provide a connectionportion with the FPC 6515 on the back side of the pixel portion, wherebyan electronic device with a narrow bezel can be obtained.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 7

In this embodiment, electronic devices each including a display devicemanufactured using one embodiment of the present invention will bedescribed.

Electronic devices illustrated below each include a display device ofone embodiment of the present invention in a display portion. Thus, theelectronic devices achieve high resolution. In addition, the electronicdevices can each achieve both high resolution and a large screen.

A display portion in an electronic device of one embodiment of thepresent invention can display an image with a resolution of, forexample, full high definition, 4K2K, 8K4K, 16K8K, or higher.

Examples of the electronic devices include a digital camera, a digitalvideo camera, a digital photo frame, a cellular phone, a portable gamemachine, a portable information terminal, and an audio reproducingdevice, in addition to electronic devices with comparatively largescreens, such as a television device, a notebook personal computer, amonitor device, digital signage, a pachinko machine, and a game machine.

An electronic device to which one embodiment of the present invention isapplied can be incorporated along a flat surface or a curved surface ofan inside wall or an outside wall of a house or a building, an interioror an exterior of a car, or the like.

FIG. 18(A) is a diagram illustrating appearance of a camera 8000 towhich a finder 8100 is attached.

The camera 8000 includes a housing 8001, a display portion 8002,operation buttons 8003, a shutter button 8004, and the like. Inaddition, a detachable lens 8006 is attached to the camera 8000.

Note that the lens 8006 and the housing may be integrated with eachother in the camera 8000.

The camera 8000 can take images by the press of the shutter button 8004or touch on the display portion 8002 functioning as a touch panel.

The housing 8001 includes a mount including an electrode, so that thefinder 8100, a stroboscope, or the like can be connected to the housing.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 is attached to the camera 8000 with a mount engagingwith a mount of the camera 8000. In the finder 8100, an image or thelike received from the camera 8000 can be displayed on the displayportion 8102.

The button 8103 has a function of a power button or the like.

The display device of one embodiment of the present invention can beapplied to the display portion 8002 of the camera 8000 and the displayportion 8102 of the finder 8100. Note that a finder may be incorporatedin the camera 8000.

FIG. 18(B) is a diagram illustrating appearance of a head-mounteddisplay 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. In addition, a battery 8206 is incorporated in the mountingportion 8201.

The cable 8205 supplies power from the battery 8206 to the main body8203. The main body 8203 includes a wireless receiver or the like andcan display received image data on the display portion 8204. Inaddition, the main body 8203 is provided with a camera, and data on themovement of the user's eyeball and eyelid can be used as an input means.

In addition, the mounting portion 8201 may be provided with a pluralityof electrodes capable of sensing current flowing in response to themovement of the user's eyeball in a position in contact with the user tohave a function of recognizing the user's sight line. Furthermore, themounting portion 8201 may have a function of monitoring the user's pulsewith the use of current flowing through the electrodes. Moreover, themounting portion 8201 may include a variety of sensors such as atemperature sensor, a pressure sensor, and an acceleration sensor tohave a function of displaying the user's biological data on the displayportion 8204 or a function of changing an image displayed on the displayportion 8204 in accordance with the movement of the user's head.

The display device of one embodiment of the present invention can beapplied to the display portion 8204.

FIGS. 18(C), 18(D), and 18(E) are diagrams illustrating appearance of ahead-mounted display 8300. The head-mounted display 8300 includes ahousing 8301, a display portion 8302, band-shaped fixing units 8304, anda pair of lenses 8305.

A user can see display on the display portion 8302 through the lenses8305. Note that the display portion 8302 is preferably curved and placedbecause the user can feel a high realistic sensation. In addition, whenanother image displayed in a different region of the display portion8302 is viewed through the lenses 8305, 3D display using parallax or thelike can also be performed. Note that the structure is not limited tothat in which one display portion 8302 is provided, and two displayportions 8302 may be provided so that one display portion is providedfor one eye of the user.

Note that the display device of one embodiment of the present inventioncan be applied to the display portion 8302. A display device including asemiconductor device of one embodiment of the present invention hasextremely high resolution; thus, even when an image is magnified usingthe lenses 8305 as illustrated in FIG. 18(E), the user does not perceivepixels, and a more realistic image can be displayed.

Electronic devices illustrated in FIG. 19(A) to FIG. 19(G) include ahousing 9000, a display portion 9001, a speaker 9003, an operation key9005 (including a power switch or an operation switch), a connectionterminal 9006, a sensor 9007 (a sensor having a function of measuringforce, displacement, a position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, power, radiation, flow rate, humidity, a gradient, oscillation,an odor, or infrared rays), a microphone 9008, and the like.

The electronic devices illustrated in FIG. 19(A) to FIG. 19(G) have avariety of functions. For example, the electronic devices can have afunction of displaying a variety of data (a still image, a moving image,a text image, and the like) on the display portion, a touch panelfunction, a function of displaying a calendar, date, time, and the like,a function of controlling processing with a variety of software(programs), a wireless communication function, a function of reading outand processing a program or data stored in a recording medium, and thelike. Note that the functions of the electronic devices are not limitedthereto, and the electronic devices can have a variety of functions. Theelectronic devices may include a plurality of display portions. Inaddition, the electronic devices may each include a camera or the likeand have a function of taking a still image or a moving image andstoring the taken image in a recording medium (an external recordingmedium or a recording medium incorporated in the camera), a function ofdisplaying the taken image on the display portion, or the like.

The details of the electronic devices illustrated in FIG. 19(A) to FIG.19(G) are described below.

FIG. 19(A) is a perspective view illustrating a television device 9100.The display portion 9001 having a large screen size of, for example,larger than or equal to 50 inches or larger than or equal to 100 inchescan be incorporated in the television device 9100.

FIG. 19(B) is a perspective view illustrating a portable informationterminal 9101. For example, the portable information terminal 9101 canbe used as a smartphone. Note that the portable information terminal9101 may be provided with the speaker 9003, the connection terminal9006, the sensor 9007, or the like. In addition, the portableinformation terminal 9101 can display characters and image informationon its plurality of surfaces. FIG. 19(B) illustrates an example in whichthree icons 9050 are displayed. Furthermore, information 9051 indicatedby dashed rectangles can be displayed on another surface of the displayportion 9001. Examples of the information 9051 include notification ofreception of an e-mail, SNS, or an incoming call, the title and senderof an e-mail, SNS, or the like, the date, the time, remaining battery,and the reception strength of an antenna. Alternatively, the icon 9050or the like may be displayed in a position where the information 9051 isdisplayed.

FIG. 19(C) is a perspective view illustrating a portable informationterminal 9102. The portable information terminal 9102 has a function ofdisplaying information on three or more surfaces of the display portion9001. Here, an example in which information 9052, information 9053, andinformation 9054 are displayed on different surfaces is illustrated. Forexample, the user can also check the information 9053 displayed in aposition that can be observed from above the portable informationterminal 9102, with the portable information terminal 9102 put in abreast pocket of his/her clothes. The user can see the display withouttaking out the portable information terminal 9102 from the pocket anddecide whether to answer a call, for example.

FIG. 19(D) is a perspective view illustrating a watch-type portableinformation terminal 9200. For example, the portable informationterminal 9200 can be used as a smart watch. In addition, a displaysurface of the display portion 9001 is curved and provided, and displaycan be performed along the curved display surface. Furthermore,intercommunication between the portable information terminal 9200 and,for example, a headset capable of wireless communication enableshands-free calling. Moreover, with the connection terminal 9006, theportable information terminal 9200 can also perform mutual datatransmission with another information terminal and charging. Note thatcharging operation may be performed by wireless power feeding.

FIGS. 19(E), 19(F), and 19(G) are perspective views illustrating afoldable portable information terminal 9201. In addition, FIG. 19(E) isa perspective view of an unfolded state of the portable informationterminal 9201, FIG. 19(G) is a perspective view of a folded statethereof, and FIG. 19(F) is a perspective view of a state in the middleof change from one of FIG. 19(E) and FIG. 19(G) to the other. Theportable information terminal 9201 is highly portable in the foldedstate and is highly browsable in the unfolded state because of aseamless large display region. The display portion 9001 of the portableinformation terminal 9201 is supported by three housings 9000 joinedwith hinges 9055. For example, the display portion 9001 can be bent witha radius of curvature greater than or equal to 1 mm and less than orequal to 150 mm.

FIG. 20(A) illustrates an example of a television device. In atelevision device 7100, a display portion 7500 is incorporated in ahousing 7101. Here, a structure in which the housing 7101 is supportedby a stand 7103 is illustrated.

Operation of the television device 7100 illustrated in FIG. 20(A) can beperformed with an operation switch provided in the housing 7101 or aseparate remote control 7111. Alternatively, a touch panel may beapplied to the display portion 7500, and the television device 7100 maybe operated by touch on the touch panel. The remote control 7111 mayinclude a display portion in addition to operation buttons.

Note that the television device 7100 may include a television receiverand a communication device for network connection.

FIG. 20(B) illustrates a notebook personal computer 7200. The notebookpersonal computer 7200 includes a housing 7211, a keyboard 7212, apointing device 7213, an external connection port 7214, and the like.The display portion 7500 is incorporated in the housing 7211.

FIGS. 20(C) and 20(D) illustrate examples of digital signage.

Digital signage 7300 illustrated in FIG. 20(C) includes a housing 7301,the display portion 7500, a speaker 7303, and the like. Furthermore, thedigital signage can include an LED lamp, operation keys (including apower switch or an operation switch), a connection terminal, a varietyof sensors, a microphone, and the like.

In addition, FIG. 20(D) is digital signage 7400 attached to acylindrical pillar 7401. The digital signage 7400 includes the displayportion 7500 provided along a curved surface of the pillar 7401.

The larger display portion 7500 can increase the amount of informationthat can be provided at a time and attracts more attention, so that theeffect of advertising can be increased, for example.

A touch panel is preferably applied to the display portion 7500 so thatthe user can operate the digital signage. Thus, the digital signage canbe used not only for advertising but also for providing information thatthe user needs, such as route information, traffic information, andguidance information on a commercial facility.

In addition, as illustrated in FIGS. 20(C) and 20(D), it is preferablethat the digital signage 7300 or the digital signage 7400 can work withan information terminal 7311 such as a user's smartphone throughwireless communication. For example, information of an advertisementdisplayed on the display portion 7500 can be displayed on a screen ofthe information terminal 7311, or display on the display portion 7500can be switched by operation of the information terminal 7311.

Furthermore, it is possible to make the digital signage 7300 or thedigital signage 7400 execute a game with the use of the informationterminal 7311 as an operation means (controller). Thus, an unspecifiednumber of users can join in and enjoy the game concurrently.

The display device of one embodiment of the present invention can beapplied to the display portion 7500 in FIGS. 20(A) to 20(D).

The electronic devices in this embodiment each have a structureincluding a display portion; however, one embodiment of the presentinvention can also be applied to an electronic device that does notinclude a display portion.

At least part of this embodiment can be implemented in combination withthe other embodiments described in this specification as appropriate.

Example 1

In this example, a sample (Sample A1) in which an impurity element wasadded to a metal oxide film and a sample (Sample A2) in which hydrogenwas supplied to a metal oxide film were fabricated and sheet resistanceof the samples was measured.

[Fabrication of Sample]

<Sample A1>

Sample A1 is a sample including an island-shaped metal oxide film over afirst silicon oxynitride film formed over a glass substrate, in whichboron was added to the metal oxide film by an ion doping method througha second silicon oxynitride film. The second silicon oxynitride film wasremoved after the addition of boron, the metal oxide film was covered bya third silicon oxynitride film, an opening reaching the metal oxidefilm was then formed, and a terminal was provided.

An In—Ga—Zn oxide with a thickness of approximately 40 nm was used asthe metal oxide film in Sample A1. The boron adding conditions were setsuch that the concentration became the highest in the vicinity of aninterface between the metal oxide film and the second silicon oxynitridefilm.

<Sample A2>

Sample A2 is a sample including an island-shaped metal oxide film over afirst silicon oxynitride film formed over a glass substrate, in whichheat treatment was performed after a silicon nitride film containinghydrogen and a fourth silicon oxynitride film were formed in contactwith the metal oxide film. After the fourth silicon oxynitride film wasformed, an opening reaching the metal oxide film was formed and aterminal was provided.

The metal oxide film in Sample A2 was formed under the same conditionsas those in Sample A1. The silicon nitride film was formed by a plasmaCVD method using a mixed gas of an SiH₄ gas, an N₂ gas, and an NH₃ gasas a deposition gas. The heat treatment was performed in a nitrogenatmosphere at 350° C. for one hour.

[Sheet Resistance]

FIG. 21 shows measurement results of the sheet resistance of fabricatedSample A1 and Sample A2.

The sheet resistance values of Sample A1 and Sample A2 are approximately380 Ω/square and approximately 240 Ω/square, respectively, as shown inFIG. 21 , which indicates that both of the resistances are reduced. Notethat the sheet resistance value of a metal oxide film whose carrierconcentration is sufficiently reduced without treatment for reducingresistance is higher than or equal to the upper detection limit of themeasurement unit (e.g., higher than or equal to 5M Ω/square).

Sample A2 to which hydrogen was supplied has lower resistance thanSample A1 to which the impurity element was added. This suggests thataddition of an impurity element and supply of hydrogen to a metal oxidefilm enable a higher carrier density and lower resistance than Sample A1and Sample A2.

The above results indicate that, in the transistor of one embodiment ofthe present invention, LDD regions having relatively high resistance canbe obtained when a metal oxide to which an impurity element is added isused for the second regions between which the channel formation region(first region) is sandwiched, and extremely low-resistance source anddrain regions can be obtained when hydrogen is supplied to the thirdregions, which are outside the second regions, after the impurityelement is added.

Example 2

In this example, a sample (Sample B1) in which an aluminum nitride filmwas formed in contact with a metal oxide film so as to reduce resistancewas fabricated and sheet resistance of the sample was measured.

[Fabrication of Sample]

<Sample B1>

Sample B1 is a sample including an island-shaped metal oxide film over afirst silicon oxynitride film formed over a glass substrate, in whichfirst heat treatment was performed after an aluminum nitride film wasformed in contact with the metal oxide film. Furthermore, an aluminumoxide film and a second silicon oxynitride film were formed after thefirst heat treatment, second heat treatment was performed, an openingreaching the metal oxide film was then formed, and a terminal wasprovided.

The metal oxide film in Sample B1 was formed under the same conditionsas those in Sample A1, which were described in Example 1 as an example.The aluminum oxide film was formed by a reactive sputtering method usingan aluminum target and using a mixed gas of an N2 gas and an Ar gas as adeposition gas. The first heat treatment and the second heat treatmentwere each performed in a nitrogen atmosphere at 350° C. for one hour.

[Sheet Resistance]

FIG. 22 shows measurement results of the sheet resistance of Sample A1described in Example 1 as an example and fabricated Sample B1.

The sheet resistance values of Sample A1 and Sample B1 are approximately380 Ω/square and approximately 270 Ω/square, respectively, as shown inFIG. 22 , which indicates that both of the resistances are reduced. Notethat the sheet resistance value of a metal oxide film whose carrierconcentration is sufficiently reduced without treatment for reducingresistance is higher than or equal to the upper detection limit of themeasurement unit (e.g., higher than or equal to 5M Ω/square).

Sample B1 in which the aluminum nitride film was formed in contact withthe metal oxide film has lower resistance than Sample A1 to which theimpurity element was added. This suggests that addition of an impurityelement to a metal oxide film and formation of a film which is likely toabsorb oxygen, such as an aluminum nitride film, in contact with themetal oxide film enable a higher carrier density and lower resistancethan Sample A1 and Sample B1.

The above results indicate that, in the transistor of one embodiment ofthe present invention, LDD regions having relatively high resistance canbe obtained when a metal oxide to which an impurity element is added isused for the second regions between which the channel formation region(first region) is sandwiched, and extremely low-resistance source regionand drain region can be obtained when a film which is likely to absorboxygen is formed over the third regions, which are outside the secondregions.

REFERENCE NUMERALS

100, 100A: transistor, 102: substrate, 103: insulating layer, 103 d:region, 106: conductive layer, 108, 108 a, 108 b: semiconductor layer,108L, 108N: region, 110: insulating layer, 110 d: region, 110 f:insulating film, 112: conductive layer, 112 f: conductive film, 114:metal oxide layer, 114 f: metal oxide film, 115: resist mask, 116, 118:insulating layer, 120 a, 120 b: conductive layer, 140: impurity element,141 a, 141 b, 142: opening

The invention claimed is:
 1. A semiconductor device comprising: asemiconductor layer; a first insulating layer; a second insulatinglayer; a third insulating layer; a first conductive layer; a secondconductive layer; and a third conductive layer, wherein the firstinsulating layer is over the semiconductor layer, wherein the firstconductive layer is over the first insulating layer, wherein the secondinsulating layer is over the semiconductor layer, the first insulatinglayer, and the first conductive layer, wherein the semiconductor layercomprises a first region that overlaps with the first conductive layerand the first insulating layer, a second region that does not overlapwith the first conductive layer and overlaps with the first insulatinglayer, and a third region that overlaps with neither the firstconductive layer nor the first insulating layer, wherein the secondinsulating layer is in contact with a top surface of the third regionand a side surface of the third region, wherein the semiconductor layeris an oxide semiconductor layer, wherein the second insulating layercomprises more hydrogen than the first insulating layer, wherein thesemiconductor layer is over the third insulating layer, wherein the topsurface of the third region is in contact with the second conductivelayer and the third conductive layer, wherein the third insulating layercomprises a fourth region that overlaps with the second conductive layerand the third conductive layer, wherein the second region, the thirdregion, and the fourth region comprise a first element, wherein thefirst element is one or more elements selected from boron, phosphorus,aluminum, and magnesium, and wherein the first element exists in a stateof being bonded to oxygen.
 2. The semiconductor device according toclaim 1, wherein the first insulating layer comprises a fifth regionthat overlaps with the first conductive layer and the first region, anda sixth region that overlaps with the second region, wherein the sixthregion comprises the first element, and wherein the first element in thesixth region exists in a state of being bonded to oxygen.
 3. Thesemiconductor device according to claim 1, wherein the first insulatinglayer comprises oxide.
 4. The semiconductor device according to claim 1,wherein the second insulating layer comprises a nitride.
 5. Thesemiconductor device according to claim 1, wherein the first insulatinglayer comprises a portion projected beyond a side surface of the firstconductive layer, and wherein an end portion of the first conductivelayer is located inward from an end portion of the first insulatinglayer in a plan view.
 6. The semiconductor device according to claim 1,wherein the second insulating layer is in contact with a top surface anda side surface of the first conductive layer, and a top surface and aside surface of the first insulating layer.
 7. The semiconductor deviceaccording to claim 1, further comprising a fourth conductive layer,wherein the third insulating layer covers the fourth conductive layer,and wherein the fourth conductive layer comprises a portion thatoverlaps with the semiconductor layer, the first insulating layer, andthe first conductive layer with the third insulating layer therebetween.